Summary

• Adds docs/deprecation-policy.md (SEP-2596): the Active → Deprecated → Removed lifecycle, the twelve-month window, and the @deprecated tag convention already in use since SEP-2577 ([Schema][Client][Server] Deprecate Roots, Sampling and Logging (SEP-2577) #427). Closes [Docs] SEP-2596: Adopt MCP feature lifecycle policy; document PHP SDK BC promise mirroring 12-month spec window #366.
• Adds docs/sdk-tier.md (SEP-1730): gap analysis against the Tier 2 bar. Server conformance is ~99%/98%; client conformance is only 20%/5% — flagged as the real gap, not a docs one. Closes [Meta] SEP-1730: Target SDK Tier 2 classification — gap analysis, roadmap, dependency policy #368.
• Wires both into mkdocs.yml's nav under a new Project section, and links them from index.md's "Where to go next". Fixes two links in sdk-tier.md that pointed at repo-root files via a path escaping docs/ — dead under Zensical's --strict build ([Docs] Render the guides with Zensical instead of phpDocumentor #414); pointed them at GitHub URLs instead, matching the existing convention.
• Adds CONTRIBUTING.md at the repo root — dev setup, the make ci checks to run before a PR, coding standards, testing expectations, the docs-guide link-resolution rule Zensical enforces, and licensing terms. Linked from README's Contributing section.

Test plan

• make docs-guides (Zensical --strict build) — no issues found
• vendor/bin/phpstan analyse — no errors
• vendor/bin/php-cs-fixer fix --dry-run --diff — 0 files need fixing