feat(agent): support repo_context_files on LocalGitProvider via shim

[huhamhire]

What

pr-agent 0.39.0 introduced a new upstream default in configuration.toml:

repo_context_files = ["AGENTS.md"]
repo_context_from_default_branch = true
repo_context_max_lines = 500

The feature fetches these files from the reviewed repo and injects them as <instruction_files> so /review, /describe, and /improve follow the project's own conventions. But LocalGitProvider (the only provider meebox uses) inherits the base no-op get_repo_file_content → _provider_supports_repo_context judges it unsupported, logs a per-run WARNING, and silently skips the feature.

Change

Add LocalGitProvider.get_repo_file_content in the version-guarded shim (local_git_provider.py):

• Reads the blob straight from the base branch's tree object (git show <target_branch>:<path>), never the working tree — the review diff is already head.commit vs merge-base(target_branch), and target_branch_name is the trusted branch the PR merges into (matches repo_context_from_default_branch=true).
• Reading the tree (not the working tree) keeps it independent of _prepare_repo's /ask-worktree sanitizing of agent instruction files.
• A missing file / any git error degrades to "", so build_repo_context treats it as "no context" and caches no fetch error.

Docs: added the patch to the current-patches list in docs/arch/02-agent/05-pragent-runtime.md.

Effect

• /review /describe /improve inject the reviewed repo's AGENTS.md (capped at repo_context_max_lines = 500) as <instruction_files>; the WARNING no longer fires.
• Token cost: only when the reviewed repo actually ships the file (else one failed git show, near-zero). When present, bounded by the line cap; the 15-min cache saves fetch/render, not prompt tokens.

Verification

• prepare:pragent re-synced the shim + smoke test passed (shim patch active).
• Ran the embedded Python directly: get_repo_file_content overrides the base no-op; existing file returns content, missing file returns ""; _provider_supports_repo_context now returns True.

The embedded shim is pure Python and not part of npm/Nx, so the root lint/typecheck/test/build do not cover it.

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