[cc] add CHANGELOG.md following keep-a-changelog format

[anipotts]

Source: Apr 2026 conventions audit

Problem

cc has no CHANGELOG.md. Marketplace publishers benefit from a structured changelog because:

1. Users see what changed when reviewing `claude plugin update cc`
2. Future cc-using skills/plugins can declare version pins
3. Anthropic's plugin marketplace UX may surface CHANGELOG content

Fix

`plugins/cc/CHANGELOG.md` following keep-a-changelog format. Backfill from git history:

```markdown

Changelog

[3.1.0] - 2026-04-28

Changed

• Single `cc` MCP tool with action-discriminated args (was 5 separate tools)
• Slash command moved to skill (`skills/sessions/SKILL.md`)
• Lifecycle unifies heartbeat/tail/watcher/db tear-down

Added

• TTL cache for liveSessions + recentFilesFor (prompt-cache stability)
• 35 unit tests for action / cache / lifecycle

Removed

• `subscribe`, `unsubscribe`, `ask`, `answer` verbs (never wired)
• `cc_cleanup` from MCP surface (still runs on shutdown)

Deprecated

• Legacy tool names `cc_sessions` / `cc_send` / `cc_announce` / `cc_check` still accepted with stderr warning; removed in v3.2

[3.0.1] - 2026-04-28

• Plugin cache invalidation via version bump

[3.0.0] - 2026-04-26

• Migration from v2 verb-dispatch tool to 5 typed tools (now reverted in 3.1)
• State directory moved to `~/.claude/channels/cc/` per imessage convention
  ```

Acceptance criteria

• CHANGELOG.md present at `plugins/cc/CHANGELOG.md`
• Backfilled from git tags/commit history
• CI updates CHANGELOG on every plugin release tag

[github-actions]

@claude please triage this issue per .github/AI_REVIEW_RUBRIC.md (issue-triage section).