main: extract .watch switch arm to runWatch helper

From fleet code review 2026-06-01 (`disorganized`, CRITICAL). The
`.watch => { ... }` block (270 lines) dispatched to the watcher subcommands
(start/stop/status/list/prune/pid/restart) — a substantial chunk of
fn main's remaining inline content.

Extracted to `fn runWatch(allocator, settings, registry, parsed, stdout)`,
preserving the inner switch on `parsed.watch_action` verbatim. Passes
`parsed: cli.Parsed` directly (3 fields read).

fn main now 391 lines — down from 1311 at the start of this session
(70% reduction). The remaining inline arms are all <50 lines each:
.config, .serve (settings shim), .symbols/.replace-symbol/.insert-*/etc.
(all already call runX helpers), .status/.log/.root/.clean/.setup_model.

Zero behavior change; all 51 test binaries pass.

Author: pmarreck