feat(compile): add awf_path_prepends for chroot PATH injection

[jamesadevine]

Summary

Adds awf_path_prepends() to the CompilerExtension trait so extensions can declare directories that should be on PATH inside the AWF chroot. The Lean runtime uses this to make lean, lake, and elan discoverable without absolute paths.

Problem

After PR #354 added the $HOME/.elan AWF mount, the agent could access Lean binaries at ~/.elan/bin/lean — but lean wasn't on PATH. The agent had to do an extra step (which lean fails → ~/.elan/bin/lean -h works) before it could use Lean.

Root cause: sudo resets PATH via secure_path. AWF reconstructs PATH internally from process.env.PATH (post-sudo, missing elan). On GitHub Actions, AWF reads the $GITHUB_PATH file to recover lost paths, but no equivalent existed for ADO.

Solution

• New CompilerExtension::awf_path_prepends() trait method — extensions declare bin directories to inject into the chroot PATH
• LeanExtension returns ["$HOME/.elan/bin"]
• New generate_awf_path_step() generates a dedicated pipeline step ("Generate GITHUB_PATH file") that writes path entries to a file and sets GITHUB_PATH via ##vso[task.setvariable]
• AWF natively reads $GITHUB_PATH at startup and merges entries into the chroot PATH

Generated step (when Lean is enabled):

- bash: |
    AWF_PATH_FILE="/tmp/awf-tools/ado-path-entries"
    echo "$HOME/.elan/bin" >> "$AWF_PATH_FILE"
    echo "##vso[task.setvariable variable=GITHUB_PATH]$AWF_PATH_FILE"
  displayName: "Generate GITHUB_PATH file"

Changes

File	Change
src/compile/extensions/mod.rs	awf_path_prepends() trait method + macro dispatch
src/runtimes/lean/extension.rs	Returns ["$HOME/.elan/bin"]
src/compile/common.rs	generate_awf_path_step() + tests
src/compile/standalone.rs	Wire up replacement
src/compile/onees.rs	Wire up replacement
src/data/base.yml	{{ awf_path_step }} marker
src/data/1es-base.yml	{{ awf_path_step }} marker
src/compile/extensions/tests.rs	Trait + Lean tests
docs/extending.md	Add method to trait listing
docs/template-markers.md	Document {{ awf_path_step }}

Testing

• All 1012 tests pass
• Clippy clean (no new warnings)
• New tests for awf_path_prepends() default + Lean impl, and generate_awf_path_step() with/without Lean

[jamesadevine]

Replaced with a clean branch cherry-picked from latest main.