feat: Add plan mode to the chat agent (#638)

* feat: Add plan mode to the chat agent

Adds a plan mode: the agent investigates read-only, proposes a checklist, and
waits for approval before changing anything. On approval it runs with full
tools and checks items off as it goes. Enforcement reuses the existing
disabled_tools gate.

Includes a slash command: `/plan [on|off]` (and `/toggle plan`) to flip the
plan toggle from the chat input.

- src/tool_security.py, src/mcp_manager.py: read-only allowlist (tools + MCP).
- src/agent_loop.py, routes/chat_routes.py: union the disabled set, prepend the
  plan directive, force agent mode.
- static/: plan toggle pill, Approve & Run, dockable plan window, task-list
  checkboxes, and the /plan slash command.
- tests/test_plan_mode.py.

* Plan mode: persistent re-referenceable plan + agent write-back

Three improvements so a long plan survives a weak model and stays in reach:

1. Re-reference the plan (out-of-context fix). On the execution turn the frontend
   sends the approved checklist back (`approved_plan`); the backend pins it as a
   top-of-context `## ACTIVE PLAN` system note (kept by the context trimmer), so
   the agent can always re-read the plan instead of losing the thread on a long
   run. New `build_active_plan_note()` (unit-tested).

2. Re-open / dock the plan anytime. The plan checklist is stored per-session
   (localStorage). When a plan exists, the plan-mode button opens a small menu
   ("Show plan" / "Plan mode: On/Off") that re-opens the side-dockable plan
   window — so it can stay docked while the agent works. The window live-refreshes
   as the plan changes.

3. Agent write-back: new `update_plan` tool. The agent calls it to tick steps
   `- [x]` after finishing them, or to revise steps when the user asks. Marker
   tool (no I/O) → `plan_update` SSE event → the stored plan + docked window
   update live. The ACTIVE PLAN note instructs the agent to use it.

Backend: src/agent_loop.py (param + pin + note builder + emit + prompt blurb),
src/tool_execution.py (update_plan handler), routes/chat_routes.py (parse
`approved_plan`, relay `plan_update`), registration in tool_schemas / agent_tools
/ tool_index (always-available, not admin-gated).
Frontend: static/js/chat.js (plan store, send `approved_plan`, handle
`plan_update`, capture restated checklists), static/app.js (plan-button menu),
static/js/planWindow.js (`isPlanWindowOpen`), static/js/storage.js (PLAN key).
Tests: tests/test_plan_mode.py (plan-note), tests/test_update_plan_tool.py.

* Plan mode: drop bash/python, rely on read-only discovery tools

Shell can mutate (write files, hit the network) and can't be constrained to
read-only at the tool layer, so plan mode no longer relies on a prompt to keep
it well-behaved — bash/python are removed from the read-only allowlist and added
to the fail-closed block set. Discovery is covered by the dedicated read-only
tools (read_file, grep, glob, ls) instead.

Rewrites the plan-mode directive to state shell is disabled and lists the
available read-only tools positively. Addresses review feedback on #638.

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

* Comment: note _MCP_READONLY_VERBS are prefixes not whole words

Clarifies that entries like "summar" are intentional stems matched via
startswith (covers summarise/summarize/summary), not typos. Addresses review
feedback on #638.

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

* Plan mode: clarify why gating inverts the allowlist into a denylist

Rename _PLAN_MODE_FALLBACK_BLOCK -> _PLAN_MODE_KNOWN_MUTATORS and rewrite the
comments. The tool gate is a denylist (disabled_tools); plan mode's policy is an
allowlist, so it returns the inverse (all known tool names minus the allowlist).
The static mutator set is a backstop for the schema-derived name list, which
misses XML-only tools and can fail to import. Addresses review feedback on #638.

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

* Plan mode: stop hardcoding the read-only tool list in the directive

The model is already shown its available (read-only) tools by _assemble_prompt,
which removes every disabled tool. Enumerating them again in the directive only
duplicated that list and would drift as tools change. Point at the tools listed
below instead. Addresses review feedback on #638.

src/tool_index.py

@@ -55,6 +55,8 @@ ALWAYS_AVAILABLE = frozenset({
     # Ask the user a multiple-choice question for a decision/clarification.
     # Always reachable so the agent can pause and ask at any point.
     "ask_user",
+    # Write back to the active plan (tick steps done / revise) during execution.
+    "update_plan",
 })
 
 # Tools that the Personal Assistant always has access to during scheduled
@@ -115,6 +117,7 @@ BUILTIN_TOOL_DESCRIPTIONS: Dict[str, str] = {
     "send_to_session": "Send a message to another chat. Cross-chat communication.",
     "search_chats": "Search through chat history across all sessions.",
     "ask_user": "Ask the user a multiple-choice question to get a decision or clarification. Use this when the task is genuinely ambiguous and the answer changes what you do next — pick between approaches, confirm an assumption, choose among options — instead of guessing. Provide a clear `question` and 2-6 `options` (each with a short `label`, optional `description`). Calling this ENDS your turn: the user sees clickable buttons and their choice arrives as your next message. Don't use it for things you can decide from context or sensible defaults, or for irreversible-action confirmation if a dedicated flow exists.",
+    "update_plan": "Write back to the ACTIVE PLAN while executing an approved plan: mark steps done or revise them. After finishing a step call this with the full checklist and that step marked done; when the user asks to change the plan call it with the revised checklist. Always pass the COMPLETE markdown checklist (`- [ ]` / `- [x]`), not a diff. The user's docked plan window updates live. No effect when there is no active plan.",
     "ui_control": "Control the UI and toggle tools on/off. Use this to turn off / turn on / disable / enable individual tools and features: shell (bash), search (web), research, browser, documents, incognito. Open panels (documents library, gallery, email inbox, sessions, notes, memories/brain, skills, settings, cookbook) via `open_panel <name>`. Use `open_email_reply <uid> <folder> reply` to open an email reply draft document without sending. Also switches between chat/agent modes, changes the current model, and applies/creates themes.",
     "list_email_accounts": "List configured email accounts and default status. Use before reading or sending mail when the user mentions Gmail, work mail, custom domain mail, another mailbox, or asks to compare/check multiple inboxes.",
     "list_emails": "List emails for a folder/account, newest first, including read messages by default. Shows subject, sender, date, UID, account, and AI summary. Check inbox, find emails needing replies. Supports account from list_email_accounts for Gmail/work/custom mailboxes. For last/latest/newest email, use max_results=1 and unread_only=false.",