Add shared-root dependency detection for downstream agent sync

[devansh-dwivedi-glean]

Description

From Glean Code Writer

Implements the shared-root change propagation feature. This enables automatic detection and re-sync of downstream agents when shared resources (skills, sub-agents, prompts, artifacts) change.

Key changes:

• action.yml: Adds shared-root input (default .glean/common/) for declaring where shared reusable agent resources live
• scripts/resolve-shared-deps.sh (new): Symlink-based dependency discovery and impact analysis engine that walks agent subtrees, resolves symlinks canonically, builds a dependency map, and computes affected agents from changed shared paths
• scripts/detect-changes.sh: Extended to partition changed files into agent-root vs shared-root, call the resolver when shared files change, and union/deduplicate direct + indirect agent triggers
• README.md: Documents shared-root usage, repo layout example, and dependency contract

Edge cases handled:

• Broken symlinks: warn, don't count as dependency
• Out-of-repo symlinks: rejected
• Directory symlinks: descendant matching
• Nested symlink chains: canonical resolution
• Overlapping roots: rejected

New step outputs: shared_root_affected, shared_changed_files for downstream observability.

Testing

12-case integration test suite in tests/test-resolve-shared-deps.sh:

• File symlink exact match
• Directory symlink with nested file change
• Multiple agents affected by one shared change
• No dependents / empty input → empty output
• Broken symlink warning (not counted as dependency)
• Symlink escaping repo → rejected with exit 1
• Overlapping roots → rejected with exit 1
• Nested symlinks resolved through chain
• Multiple shared changes affecting different agents
• Symlinks outside shared_root ignored

All 12 tests pass.

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