fix(coding-agent): verify pinned checkout contents

[brownie-ricon]

Summary

• require pinned Git checkouts to match both the configured commit and a clean worktree before loading resources
• reset and clean matching-HEAD checkouts when tracked or untracked files differ
• skip dirty pinned checkouts offline
• make the matching-pin test assert the local-only Git commands and update the temporary-package fixture for the integrity check

Validation

• npm run check
• GIT_CONFIG_COUNT=1 GIT_CONFIG_KEY_0=tag.gpgSign GIT_CONFIG_VALUE_0=false node node_modules/vitest/vitest.mjs --run test/package-manager.test.ts test/git-update.test.ts from packages/coding-agent: 134 passed

Targets #22's branch so Junior can accept or reject the correction independently.

This comment is AI-generated by Brownie.