fix(tui): wire up features that were built but never reachable

[emal-avala]

Summary

A TUI audit turned up several capabilities that exist in the tree but are never actually connected — code that is written, sometimes tested, and unreachable at runtime. This wires them up and pins each contract with tests.

What was dead, and what it does now

Feature	Before	After
Ctrl+L	Documented in KEYBINDINGS.md, no handler existed	Forces a full repaint (invalidates the layout cache + clears the terminal)
Kitty keyboard protocol	Detected and reported, but PushKeyboardEnhancementFlags was never called anywhere in the repo — so the Ctrl+Enter/Shift+Enter disambiguation the UI advertises was never requested	Pushed on entry, popped on exit / restore / panic, balanced so we never pop flags we didn't push
Desktop notifications	NotifierService fully implemented and tested, never constructed	Notifies when attention is required and the terminal is unfocused: permission/plan/question modal opens, and turn completion
NO_COLOR	Mapped to 16-colour — i.e. still colour, contrary to no-color.org	Real monochrome mode: drops fg/bg, preserves bold/dim/italic/underline so structure survives. Also honours CLICOLOR=0; FORCE_COLOR can override upward
Non-TTY launch	Died inside enable_raw_mode() with a raw errno	Fails early naming the offending stream and pointing at --prompt / --output-format json

Notes on the trickier two

Kitty protocol is deliberately not enabled on browser-engine terminal widgets embedded in code editors — they mis-encode shifted printables under this protocol. The decision is a pure function (keyboard_enhancement_allowed) so it's unit-tested rather than inferred at runtime. Push/pop ordering matches the existing teardown discipline exactly (pushed last, popped first, including in the panic hook) — a leaked keyboard mode wrecks the user's shell.

Notifications are dispatched off the event loop via spawn_blocking: the Linux/Windows backends do a synchronous which/where.exe probe and the macOS focus probe shells out, so dispatching inline would stall rendering. The gate uses the terminal's own focus-change reporting rather than the service's probe, which always returns false on Linux/Windows.

A suspected bug that turned out not to exist

The audit flagged /tasks as shadowed by the pane toggle, making the background-task CLI unreachable. Investigated: false. The line in question is a skill-shadow skip list (stopping a user skill named tasks from hijacking the command); the toggle matches bare /tasks exactly, so /tasks output 3 already falls through to the command bridge. No code change — but a regression test now pins both halves so a future edit can't introduce the bug that was suspected.

Verification

• 565 tests pass (31 new), clippy --all-targets -- -D warnings clean, fmt --check clean.
• New tests cover: Ctrl+L redraw, kitty push/pop escape sequences and stack balance, the enhancement allow/deny matrix, the TTY-guard matrix and message actionability, NO_COLOR/CLICOLOR/FORCE_COLOR precedence, mono attribute preservation, per-slot palette stripping, notifier decision logic, and the /tasks routing contract.
• One existing test changed deliberately: no_color_overrides_apple_terminal now expects mono instead of 16-colour — that is the fix.

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