Materialize task-level context fixtures in workspaces

[Copilot]

copilot-sdk runs were accepting inputs.context.fixture as task metadata but never copying those fixture files into the per-task workspace. Agents started in an empty temp directory even when task YAML declared a fixture.

• Workspace materialization

  • Expands inputs.context.fixture into execution resources before engine execution.
  • Supports fixture files and directories.
  • Copies directory fixture contents into the workspace root.

• Path safety

  • Rejects empty, absolute, and traversal fixture paths.
  • Keeps existing inputs.files, instruction files, and git resource behavior unchanged.

• Coverage and docs

  • Adds regression coverage for directory fixture copying and traversal rejection.
  • Documents the supported inputs.context.fixture behavior in README, guide docs, site docs, and schema metadata.

Example:

inputs:
  context:
    fixture: fixtures/demo
  prompt: "Inspect repository files and summarize what you find."

Files under fixtures/demo are now available in the fresh task workspace before the agent runs.

[Copilot]

Pull request overview

This PR makes inputs.context.fixture in task YAML actually materialize fixture content into each task’s fresh workspace before engine execution, aligning Copilot SDK runs with the declared task context and documenting the behavior across docs and schema metadata.

Changes:

• Adds task-level fixture materialization (inputs.context.fixture) supporting both files and directories (directory contents copied into workspace root).
• Introduces path-safety validation for fixture paths (rejecting empty/absolute/traversal) and adds regression tests for directory copying + traversal rejection.
• Updates README, guide docs, and the task schema description to document the new supported behavior.

Show a summary per file

File	Description
site/src/content/docs/guides/eval-yaml.mdx	Documents inputs.context.fixture and directory-copy semantics in site docs.
schemas/task.schema.json	Updates schema metadata to mention fixture behavior (recommend defining fixture explicitly).
README.md	Adds user-facing documentation for task fixture workspace materialization.
internal/orchestration/runner.go	Implements fixture-to-workspace resource expansion before execution.
internal/orchestration/runner_test.go	Adds tests for directory fixture materialization and traversal rejection.
docs/GUIDE.md	Adds guide documentation for using inputs.context.fixture.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 4