security(deps): add upper bounds to 5 loose deps + document supply chain policy (#24226)

After the Mini Shai-Hulud supply chain campaign (May 2026) and the litellm
compromise (March 2026), codify the dependency pinning policy that was
established in PRs #2810 and #9801 but never written down for contributors.

Changes:
- pyproject.toml: Add tight upper bounds to the 5 deps that slipped
  through as review escapes from external contributor PRs:
  - hindsight-client>=0.4.22,<0.5 (was >=0.4.22)
  - aiosqlite>=0.20,<0.23 (was >=0.20)
  - asyncpg>=0.29,<0.32 (was >=0.29)
  - alibabacloud-dingtalk>=2.0.0,<3 (was >=2.0.0)
  - youtube-transcript-api>=1.2.0,<2 (was >=1.2.0)

  Pre-1.0 packages get <0.(current_minor+2) — tight enough to block
  hostile minor releases but loose enough to not require bumps every week.

- CONTRIBUTING.md: Add 'Dependency pinning policy' section under Security
  with the full rationale, table of source types + treatments, and examples.

- AGENTS.md: Add concise 'Dependency Pinning Policy' section for AI coding
  agents with the decision table and step-by-step checklist.

- supply-chain-audit.yml: Add dep-bounds job that fails PRs introducing
  PyPI deps without <ceiling upper bounds. Fires on pyproject.toml changes.
  Posts a PR comment with the specific unbounded specs found.

Refs: #2796 #2810 #9801 #24205

AGENTS.md

@@ -308,6 +308,29 @@ The registry handles schema collection, dispatch, availability checking, and err
 
 ---
 
+## Dependency Pinning Policy
+
+All dependencies must have upper bounds to limit supply-chain attack surface.
+This policy was established after the litellm compromise (PR #2796, #2810) and
+reinforced after the Mini Shai-Hulud worm campaign (May 2026).
+
+| Source type | Treatment | Example |
+|---|---|---|
+| PyPI package | `>=floor,<next_major` | `"httpx>=0.28.1,<1"` |
+| Git URL | Commit SHA | `git+https://...@<40-char-sha>` |
+| GitHub Actions | Commit SHA + comment | `uses: actions/checkout@<sha>  # v4` |
+| CI-only pip | `==exact` | `pyyaml==6.0.2` |
+
+**When adding a new dependency to `pyproject.toml`:**
+1. Pin to `>=current_version,<next_major` for post-1.0 (e.g. `>=1.5.0,<2`).
+2. For pre-1.0 packages, use `<0.(current_minor + 2)` (e.g. `>=0.29,<0.32`).
+3. Never commit a bare `>=X.Y.Z` without a ceiling — CI and reviewers will reject it.
+4. Run `uv lock` to regenerate `uv.lock` with hashes.
+
+Reference: #2810 (bounds pass), #9801 (SHA pinning + audit CI).
+
+---
+
 ## Adding Configuration
 
 ### config.yaml options: