✨ feat: formalize shared Ghostty sessions

[zrr1999]

Summary

• make cued the sole owner of PTYs, processes, named sessions, raw history, and controller leases
• support durable shared human/agent sessions with multiple observers, a single controller, explicit handoff, generation fencing, archive/restore, and reconnect
• hard-cut the TUI to the single cue-terminal / libghostty-vt backend and remove the legacy vt100 / tui-term path
• bound foreground input, terminal reply, client writer, and UI event queues; fail closed on stale leases, oversized terminal transactions, partial writes, and failed detach
• make pending connection attempts switchable/cancellable and apply an end-to-end connection timeout
• pin the Ghostty source, Zig toolchain, minimal VT-only patch, verified dependency closure, licenses, static wheel packaging, and cross-platform package CI
• document the formal TUI ownership model and the Spark integration boundary through typed cue IPC rather than duplicated PTY/session state

Why

cue-shell's primary Zellij replacement use case is a named process session that humans and agents can safely observe and share. The previous client-side terminal path duplicated terminal behavior and did not make authority, replay, or control handoff explicit.

This change keeps execution truth in the daemon while treating each client terminal as a reconstructable projection. It intentionally does not introduce cmux, a second terminal backend, or a separate session owner.

User and developer impact

• humans and agents can attach to the same named session without both writing concurrently
• observers retain local scrolling, selection, and copy while only the confirmed controller can send input, terminal replies, or resize events
• reconnect and target switching no longer wait indefinitely behind a stuck connector
• Ghostty integration is static, pinned, reproducible, and fails closed when the prepared dependency closure is unavailable
• Spark integrates through the existing typed JSON IPC/session contract and does not link Ghostty or duplicate cue daemon state

Validation

• just ci
  • cue-client: 137 tests
  • cue-daemon: 496 tests
  • daemon integration: 51 tests
  • cue-terminal: 18 tests
  • cue-tui: 222 tests
• just pre-commit
• just tui-debug-smoke against a real PTY
• scripts/verify-ghostty-deps.sh
• scripts/verify-ghostty-fail-closed.sh
• git diff --check origin/main...HEAD