feat: answer terminal capability queries (OSC 10/11, DSR)

[ThomasK33]

Problem

agent-tty owns the PTY master and is the terminal that programs in a session talk to, but it never replied to the terminal-capability queries programs emit at startup. So TUIs like Neovim 0.12+ time out probing for the background color and print:

E1568: Terminal did not respond to DSR request for 'background' color. Startup may be slower. :help 'ttyfast'

That warning then lands in the very PNG/WebM captures agent-tty exists to produce. (Reported while using agent-tty with Neovim.)

Fix

Add a small, stateful VT scanner — src/pty/terminalQueryResponder.ts — that answers the handful of startup probes that matter and wire it into the host's live pty.onData loop, writing replies back to the child:

• OSC 11 (background color) → replies with the render profile's background
• OSC 10 (foreground color) → replies with the foreground
• DSR ESC[5n (status) → replies ESC[0n (the sentinel programs use to learn the terminal answers queries at all)

Details:

• The scanner is a state machine (not a regex) so a query split across PTY read boundaries is still answered, and it has a fast-path that skips the per-character scan for the (overwhelmingly common) chunks with no ESC byte.
• Advertised colors mirror the default render profile, so a program's light/dark choice matches a default-profile capture. The probe is answered once at startup, before any capture profile is known.
• Replies are written as child input and are not appended to the event log, so snapshots/screenshots/recordings are unaffected. The reply write runs after queuing output ingestion, so the canonical output path is never gated on it.
• Lives in the host's live loop, not a renderer backend — the renderers are offline event-log replay consumers and never participate in the live PTY conversation.

Testing

• Unit (test/unit/pty/terminalQueryResponder.test.ts): OSC 10/11 with BEL & ST terminators, the OSC 11 + DSR startup handshake, split-across-chunk queries, embedded/multiple queries, non-query inputs (SGR, title sets, color sets, DA1, CPR), recovery after aborted sequences and after an 8-bit C1 ST, rgb conversion, constructor validation.
• Integration (test/integration/terminal-query-responder.test.ts): a child sets raw mode, emits the OSC 11 query, and reports what it received on stdin — proving the full loop (child query → host reply → child stdin).
• typecheck ✓, lint ✓, full unit suite ✓, full integration suite ✓.
• Verified against real nvim --clean v0.12.3 via the agent-tty CLI: with the responder the warning is gone; disabling it reproduces the exact E1568 line.

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