Adds Context Tier Hydration subsection to the ACMS Architecture section of
the specification, documenting the context_tier_hydrator module's public
interface (hydrate_tiers_for_plan, hydrate_tiers_from_project), file listing
strategy (git ls-files for git-checkout resources, os.walk fallback), budget
limits (256 KB per file, 10 MB total per project), and fragment structure
(TieredFragment with ContextTier.HOT placement, metadata keys path/detail_depth/relevance_score).
Also updates CHANGELOG.md under [Unreleased] > Documentation and adds
contribution entry to CONTRIBUTORS.md.
ISSUES CLOSED: #6175
HAL9000
7ba1e5737f