docs(changelog): record session-prune helper + doctor memory block

Two items added in this stabilization pass that weren't yet in the
changelog:
- SessionManager::prune_sessions_older_than (#406 phase-1)
- doctor --json memory block (#489 follow-up)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -57,6 +57,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **`docs/MEMORY.md`** — user-facing memory feature documentation.
 - **Competitive analysis doc** — `docs/COMPETITIVE_ANALYSIS.md`
   catalogues capability matrix vs OpenCode and Codex CLI.
+- **Session-prune helper** (#406 phase-1) —
+  `SessionManager::prune_sessions_older_than(max_age)` with 5 unit
+  tests pinning the contract. Building block for the v0.8.9
+  auto-archive design; currently unwired pending retention-policy
+  decisions.
+- **`deepseek doctor --json`** now surfaces a `memory` block
+  (`enabled` / `path` / `file_present`) so operators can verify
+  memory configuration without booting the TUI.
 
 ### Changed
 - **Sub-agent concurrency cap raised to 10 by default** (#509) —