docs: rewrite README and remove registry references

Rewrote the "Why" section to focus on key differentiators (version
pinning, version switching, ephemeral exec). Removed all registry
references from README and AGENTS.md since the registry was replaced
by aliases.json.

Author: lilienblum

AGENTS.md

@@ -14,13 +14,13 @@ Zig 0.15+ required. This is Zig 0.15 which uses newer APIs (`std.Io.Writer.Alloc
 
 ## Architecture
 
-Onyx is a package manager with two install paths: **registry packages** (resolved via Onyx registry, fetched from the Nix binary cache) and **third-party packages** (GitHub/domain sources with `onyx.toml` manifests, installed directly).
+Onyx is a package manager with two install paths: **Nix packages** (resolved via aliases + Nixhub API, fetched from the Nix binary cache) and **third-party packages** (GitHub/domain sources with `onyx.toml` manifests, installed directly).
 
 ### Source files
 
 - **main.zig** — CLI entry point, all command implementations (`cmdInstall`, `cmdExec`, `cmdUninstall`, `cmdUse`, `cmdGc`, `cmdUpgrade`, `cmdInit`, `cmdImplode`)
 - **cli.zig** — argument parsing, `PackageRef` (name@version), `Command` union
-- **resolver.zig** — registry alias resolution, Nixhub API calls, index caching with TTL
+- **resolver.zig** — alias resolution, Nixhub API calls, index caching with TTL
 - **fetcher.zig** — NAR closure fetching, parallel download (thread pool), hash verification
 - **source.zig** — third-party package resolution (GitHub, domain), TOML manifest parsing, meta tag discovery
 - **store.zig** — `Database` struct: state.json load/save, version management, symlink install/remove
@@ -30,7 +30,7 @@ Onyx is a package manager with two install paths: **registry packages** (resolve
 
 ### Two install paths
 
-**Nix path**: `resolveAlias` (registry index) → `resolve` (Nixhub API) → `fetchClosure` (cache.nixos.org, parallel NARs) → unpack to `/nix/store/` → symlink to `~/.local/bin/`
+**Nix path**: `resolveAlias` (aliases.json) → `resolve` (Nixhub API) → `fetchClosure` (cache.nixos.org, parallel NARs) → unpack to `/nix/store/` → symlink to `~/.local/bin/`
 
 **Third-party path**: `resolveGithub`/`resolveDomain` → download binary/tarball → install to `/opt/onyx/packages/{name}/{version}/` → symlink to `~/.local/bin/`
 
@@ -39,7 +39,7 @@ Both paths store state in `~/.local/share/onyx/state.json` and create symlinks i
 ### Key patterns
 
 - **Locking**: `acquireLock`/`releaseLock` via file lock on `~/.local/share/onyx/lock`. Commands that mutate state (`install`, `uninstall`, `use`, `gc`, `upgrade`) must hold the lock. `cmdInstall` wraps `cmdInstallInner` to avoid recursive lock acquisition when installing dependencies.
-- **Alias resolution**: `resolveAlias` checks `~/.cache/onyx/index.json`. Fast commands (`exec`, `use`, `list`) use cached index forever. Slow commands (`install`, `upgrade`) refresh if >1 day old via `resolveAliasFresh`.
+- **Alias resolution**: `resolveAlias` checks `~/.cache/onyx/aliases.json`. Fast commands (`exec`, `use`, `list`) use cached aliases forever. Slow commands (`install`, `upgrade`) refresh if >1 day old via `resolveAliasFresh`.
 - **Ephemeral packages**: `exec` auto-installs packages marked `ephemeral: true` with `last_used` timestamp. These are hidden from `list` and cleaned by `gc` after 30 days.
 - **Symlink ownership**: `removeSymlinks` uses `readLink` to verify the symlink points to `/nix/store/` or `/onyx/packages/` before deleting — never removes files owned by other tools.
 - **exec fast path**: `cmdExec` checks state.json first. If the package is already installed, it skips all network calls and directly `execv`s into the binary.

README.md

@@ -1,6 +1,6 @@
 # Onyx
 
-Onyx is a package manager that combines a curated registry with the Nix binary cache. The registry provides name resolution and aliases; the Nix cache provides 80,000+ prebuilt packages. Third-party packages skip Nix entirely — they publish an onyx.toml with download URLs and install directly to `~/.local/bin`.
+Onyx is a package manager backed by the Nix binary cache — 80,000+ prebuilt packages, installed in seconds. Third-party packages skip Nix entirely — they publish an onyx.toml with download URLs and install directly to `~/.local/bin`.
 
 
 ```bash
@@ -15,18 +15,15 @@ onyx x ruby -- script.rb
 
 ## Why
 
-| | Onyx | Homebrew | Nix | apt |
-|---|---|---|---|---|
-| Install speed | Seconds (prebuilt) | Minutes (often builds) | Seconds | Seconds |
-| Packages | 80,000+ | ~7,000 | 80,000+ | Distro-dependent |
-| Multi-version | `onyx use node@20` | No | Manual | No |
-| Cross-platform | Linux + macOS | macOS-first | Linux + macOS | Linux only |
-| Binary size | <1MB | 12MB+ Ruby runtime | 30MB+ | System package |
-| Deterministic | Yes (content-addressed) | No | Yes | No |
+Nix has the best package infrastructure ever built — 80,000+ prebuilt packages in a content-addressed binary cache.
 
-Onyx takes the best of each world:
-- **Nix's binary cache** — 80k+ packages, content-addressed, reproducible
-- **None of the complexity** — no Ruby, no Nix language, no learning curve
+Onyx gives you the cache without the overhead. One static binary under 1 MB, no configuration, no new language to learn.
+
+- **Version pinning** — `onyx install node@20` and `node@22` live side by side, each install is atomic
+- **Version switching** — `onyx use node@20` flips the active version instantly
+- **Run without installing** — `onyx x jq -- '.name' package.json` fetches and runs, `onyx gc` cleans up after 30 days
+- **Cross-platform** — same tool, same packages on Linux and macOS
+- **Extensible** — third-party packages via simple TOML manifests, no Nix expressions needed
 
 ## Quickstart
 
@@ -78,22 +75,14 @@ Third-party packages (`user:repo`, `domain.com`) skip the nix store entirely and
 
 ```mermaid
 flowchart LR
-    A[onyx install nodejs@22] --> B
-
-    subgraph B[Registry]
-        direction TB
-        B1[Resolve alias] --> B2{Onyx package?}
-    end
-
-    B2 -->|yes| D[Install to ~/.local/share/onyx/]
-    B2 -->|no| C[Nixhub API]
-    C --> E[cache.nixos.org]
-    E --> F[Install to /nix/store/]
-    D -->|symlink| G[~/.local/bin/]
-    F -->|symlink| G
+    A[onyx install nodejs@22] --> B[Resolve alias]
+    B --> C[Nixhub API]
+    C --> D[cache.nixos.org]
+    D --> E[Install to /nix/store/]
+    E -->|symlink| F[~/.local/bin/]
 ```
 
-The [Onyx registry](https://github.com/lilienblum/onyx/tree/registry/v0) is checked first. If it has a direct package definition, the binary is downloaded and installed. Otherwise, the name is resolved through the Nix path — Nixhub API, dependency closure from `cache.nixos.org`, parallel download, unpack, and symlink into `~/.local/bin/`.
+Aliases (`node` → `nodejs`) are resolved first, then the Nixhub API finds the package, its dependency closure is fetched in parallel from `cache.nixos.org`, unpacked to `/nix/store/`, and symlinked into `~/.local/bin/`.
 
 Third-party packages (`user:repo`, `domain.com`) fetch an `onyx.toml` manifest, download the binary for your platform, verify SHA256, and install directly — no nix store involved.
 
@@ -124,7 +113,7 @@ Aliases: `i` install, `rm` uninstall, `x` exec, `ls` list.
 /opt/onyx/packages/my-tool/                third-party packages
 
 ~/.local/share/onyx/state.json             package state
-~/.cache/onyx/index.json                   registry index
+~/.cache/onyx/aliases.json                 alias cache
 ```
 
 ## Third-party packages