feat(terminal): Ctrl+C copies the selection, interrupts when nothing is selected

Closes #211. Copying from the terminal only worked through the browser
context menu, because xterm turns Ctrl+C into 0x03 and cancels the keydown,
so the muscle-memory copy failed silently and read as "no copy-paste at all".

With a selection, Ctrl+C now copies it, toasts, clears the selection and
sends nothing to the PTY. With no selection it falls through unchanged, so
the interrupt is intact. Ctrl+Shift+C is an explicit copy chord that never
falls through: an explicit copy that interrupts a running agent because the
selection happened to be empty would be a footgun.

Three details that keep the interrupt safe:

- The decision lives in attachCustomKeyEventHandler (terminal-ui.js) and the
  no-selection path returns true WITHOUT preventDefault. xterm calls the
  custom handler before its own cancel(), so returning false alone does not
  cancel the event; the copy path therefore calls preventDefault explicitly,
  or the browser would run its native copy on top of ours.
- copy-selection is a full registry entry (rebindable and disableable in App
  Settings) whose action is deliberately absent from SHORTCUT_ACTIONS, the
  same trick command-palette uses: the generic capture loop preventDefaults
  every match it dispatches, which would cost the user the interrupt key.
- The gate is keydown-only, since the custom handler also runs for keypress
  and keyup.

Copy goes through _copyText (Clipboard API, then hidden-textarea +
execCommand) rather than raw navigator.clipboard, because install.sh's LAN
option serves plain HTTP where navigator.clipboard is undefined; the
fallback steals focus, so the terminal is refocused afterwards.

Tests: test/terminal-copy-selection.test.ts pins the gate and the
SHORTCUT_ACTIONS invariant; test/terminal-copy-shortcut.test.ts drives real
key presses in chromium and asserts on the clipboard plus the bytes xterm
emitted (browser-driven, so excluded from test:ci like the other Playwright
suites). Verified manually on an isolated beta instance before landing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -138,6 +138,14 @@ The general rule: **any new endpoint that turns a caller-supplied `sessionId` in
 
 **Command palette + shortcut registry** (COD-151/153/157/192, #146): `Ctrl/Cmd/Alt+K` opens the session palette (fuzzy search over live sessions; "Browse all sessions" → the Session Manager modal backed by `GET /api/sessions/unified`); the quick-start case `<select>` is fronted by a searchable picker (`buildCasePickerOptions`/`formatCasePickerLabel` — remote cases render `name @ hostId`). Shortcuts live in a rebindable registry (`DEFAULT_SHORTCUTS`/`getShortcutRegistry()`/`matchesShortcutEvent()` in app.js; overrides persist under `settings.shortcutOverrides` via `saveAppSettingsToStorage`); App Settings → Shortcuts renders capture/disable rows; `Ctrl+?` opens the registry-driven overlay (footer links to the full `#helpModal` reference). ⚠️ Palette-chord keys must ALSO be swallowed in `attachCustomKeyEventHandler` (terminal-ui.js) or xterm writes the control byte (0x0B) into the PTY. ⚠️ `saveAppSettings()` rebuilds settings from the DOM — keys edited elsewhere (`shortcutOverrides`, `showTokenCount`, `showCost`) need explicit `_prev` carry-over.
 
+**Terminal smart copy** (#211): `Ctrl+C` copies the selection when there is one and stays the interrupt when there isn't. Three rules keep that split honest, and breaking any of them silently costs the user their interrupt key:
+
+1. The branch lives in `attachCustomKeyEventHandler` (terminal-ui.js) and the **no-selection path returns `true` with no `preventDefault()`**, so xterm still evaluates `Ctrl+C` into `0x03`. Returning `false` alone does not cancel the event either way: xterm's `_keyDown` calls the custom handler *before* its own `cancel()`, which is exactly why the copy path calls `preventDefault()` explicitly (otherwise the browser also runs its native copy on top).
+2. `copyTerminalSelection` is a registry `action` **deliberately missing from `SHORTCUT_ACTIONS`** (same trick as `command-palette`): the entry stays rebindable and disableable in App Settings, while the generic document-capture loop, which `preventDefault()`s every match it dispatches, skips it and lets the terminal handler decide.
+3. The gate is keydown-only (the custom handler also runs for `keypress`/`keyup`), and `Ctrl+Shift+C` never falls through to the PTY: an "explicit copy" chord that interrupts a running agent because the selection happened to be empty is a footgun with no upside.
+
+Copy goes through `_copyText()` (Clipboard API, then hidden-textarea + `execCommand`), not raw `navigator.clipboard`, because `install.sh`'s LAN option serves plain HTTP where `navigator.clipboard` is undefined; the fallback steals focus, so the terminal is refocused afterwards. Related: xterm registers its own `copy` listener on the terminal element gated on `hasSelection()`, which is why right-click → Copy has always worked. Selection itself is unavailable on touch devices by design (`user-select: none` on the terminal subtree), and in `shell`/`opencode`/`antigravity` tabs the TUI owns the mouse, so selecting there needs Shift+drag. Tests: `test/terminal-copy-selection.test.ts` (gate + wiring invariants), `test/terminal-copy-shortcut.test.ts` (browser, real key presses).
+
 ### WebGL renderer toggle
 
 **WebGL renderer toggle** (#140, `webglRendererEnabled`): per-device (`displayKeys` set, stripped from the server payload — NOT in `SettingsUpdateSchema`, which is `.strict()`). The GPU-stall watchdog's sticky `codeman-webgl-disabled` marker survives page loads; it's cleared only by an explicit OFF→ON save transition or `?webgl=force` (`shouldSkipWebGL` in constants.js). `?nowebgl` still forces the DOM renderer per-load.