docs: track post-0.93.0 share-AI cap change + deferred stream-coverage debt

Follow-ups from the multi-aspect review of the e5bc82c7..d4658d4c range.

- CHANGELOG: document under [Unreleased] that the default per-workspace
  hourly public-share assistant cap was lowered 300 -> 100 after the
  v0.93.0 tag (#62). v0.93.0 shipped 300, so existing deployments that
  never set SHARE_AI_WORKSPACE_MAX_PER_HOUR drop to 100 on upgrade.
- Recreate the still-open Section 3 (AiChatService.stream integration
  coverage) of the deleted feature-test-coverage-deferred.md as a focused
  backlog doc so the test debt stays tracked; Sections 1-2 are already
  closed by the integration harness (PR #115).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -10,6 +10,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- **Public share AI: default per-workspace hourly assistant cap lowered
+  300 → 100.** The limiter falls back to this default whenever
+  `SHARE_AI_WORKSPACE_MAX_PER_HOUR` is unset, so a `0.93.0` deployment that
+  never set the env var has its anonymous public-share assistant hourly cap
+  cut from 300 to 100 on upgrade. Set `SHARE_AI_WORKSPACE_MAX_PER_HOUR` to
+  keep the previous limit. (#62)
+
 ## [0.93.0] - 2026-06-21
 
 This release builds on the 0.91.0 AI foundation: admin-defined AI agent roles,