feat: implement history mode and AI development logging

- add dedicated history mode UI (toggle with `h`)
- allow navigation inside history with arrows
- allow selecting history entries with Enter
- add tests for history mode behavior
- introduce AI development logs (`ai-worklog.md`, `ai-prompts.md`)
- add AI worklog policy to `instructions-agent.md`

docs/ai-worklog.md

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+# AI Worklog
+
+This file records important AI-assisted development actions.
+
+Each entry should include:
+- date
+- task type (plan/build)
+- objective
+- files inspected or modified
+- key decisions
+- validation commands executed
+- result
+- open issues (if any)
+
+Entries must be concise and chronological.
+
+## 2026-03-15 - build
+
+- objective: Replace the temporary history debug overlay with a dedicated history mode in the terminal UI.
+- files modified: `src/app/ui.py`, `tests/test_ui.py`
+- key decisions: Keep `b` and `f` unchanged, add separate history-mode selection state inside the UI, hide the tree while history mode is active, and use Enter to select a history entry and return to the tree view.
+- validation: `make fix`, `make quality`
+- result: History mode now shows only the history list, supports Up/Down selection, allows Enter to jump to a prior directory, and exits cleanly back to normal navigation.
+- open issues: None.