feat(terminal): Mouse Mode Submit button for agent terminal panes

[Juliusolsson05]

Motivation

Mouse Mode (mouseModeEnabled) gates Send/Stop in ComposerActions, but only inside the rendered feed (TileLeaf). When an agent runs in Terminal view (AgentTerminalLeaf — Claude/Codex TUI, and the OpenCode Terminal runtime), there is no composer and no mouse-reachable way to press Enter. A mouse-only user who dictated or pasted a command into the raw PTY cannot submit it. Dictation deliberately does not auto-submit ("user reviews, then presses Enter" — useComposerDictation), so a mouse-only user has no path to that final Enter without the keyboard.

Follows the Mouse Mode gap from docs/superpowers/plans/2026-07-28-mouse-first-workspace.md (PR #617, IMPLEMENTED): mouse users could not interrupt before; they still cannot submit in terminal view.

Intended behavior

• When mouseModeEnabled is on, AgentTerminalLeaf renders a thin action row below the xterm with a Submit button.
• Clicking Submit sends '\r' (Enter) to the agent PTY via the existing window.api.sendInput(sessionId, ...) path — the exact byte xterm emits for Enter.
• It must NOT read, clear, or invent any draft (runtime.draftInput and feed state are untouched). Raw terminals have no readable "current line", so the button is always enabled, like a hardware Enter key.
• onMouseDown preventDefault so the click never steals xterm focus.
• Scope: agent terminal panes only (AgentTerminalLeaf). Plain shell panes (TerminalLeaf) keep zero controls — ordinary shell behavior is preserved. Stop button is out of scope (interrupt semantics differ per TUI).

Acceptance criteria

• New colocated renderer component/test for the action row.
• Row mounts only when mouseModeEnabled; zero layout change when off.
• Submit sends '\r' to the agent PTY; no draft/composer state is touched.
• TerminalLeaf byte-identical (no control added to plain shells).
• npm run typecheck and npm run test:renderer pass.
• Open reviewable PR referencing this issue; do not merge without user confirmation.