feat(tui): Tab accepts the composer's suggested query placeholder

[ericleepi314]

Summary

The composer's dim placeholder on a fresh idle session (Try "explain this codebase", Try "/help" for commands, …) is a system suggestion of a query — but there was no way to take it. Plain Tab now accepts it: the suggested query is inserted into the input with the cursor at the end.

• Try "explain this codebase" → inserts explain this codebase
• Try "write a test for…" → inserts write a test for (ellipsis dropped, trailing space kept so the sentence is continuable)
• Try "/help" for commands → inserts /help and pops the slash completion menu exactly as if typed
• Ask me anything… carries no query → Tab stays inert

Mirrors original CC's Tab fallback (typescript/src/hooks/useTypeahead.tsx handleKeyDown: tab / no shift / no suggestions / empty input → accept the prompt suggestion); there the suggestion state holds the bare query, ours is embedded in the Try "…" placeholder string, hence the extraction helper.

Gating

shouldAcceptPlaceholderSuggestion mirrors the placeholder-visibility condition exactly (conversation empty + idle + empty input, threaded as InputHandlerContext.conversationEmpty from the same flag appLayout renders from), plus no-completions and no-shift — so Tab can never insert what isn't displayed and never shadows completion-accept or the Shift+Tab permission-mode cycle.

Verification

• 16 unit tests (extraction incl. round-trip invariant over the shipped list; all six predicate gates); tsc --noEmit and eslint clean
• Full ui-tui vitest: zero new failures (8 pre-existing environmental failures proven identical on clean main via stash/run/pop; 1 known full-suite-load flake)
• Live pyte e2e in a real PTY against the dev backend: all three placeholder families pass (plain query, ellipsis stub, /help + menu), incl. second-Tab-inert; PTY-resize repaint used to defeat cell-diff renderer artifacts in pyte
• Critic subagent: APPROVE · Codex adversarial review: approve, no material findings

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