docs: spec for inline code comments on diffs

Draft proposal for line-anchored comment threads on the commit and compare
pages. Records the key finding that storage is already available through
sr-ht-core (Postgres via the database package + connection-string), that the
service is stateless by choice rather than by limitation, and that @pierre/diffs
already provides the thread UI primitives (DiffLineAnnotation + renderAnnotation
+ onDiffLineClick). Covers the anchoring model, schema, authz reuse, HTTP API,
frontend wiring, config, and a commit-first phasing.