[codex] Fix Claude macOS PTY isolation with experimental ptyHost

[parsakhaz]

Summary

This PR implements the macOS Claude PTY fix and introduces the experimental ptyHost path behind the existing settings checkbox / PANE_USE_PTY_HOST=1 flag.

Changes include:

• Add Phase 0 Claude Unix wrapper via sh -c 'exec ...' to reduce inherited Electron process state for Claude Code v2.1.113+.
• Add main/src/ptyHost/* UtilityProcess RPC host for PTY spawn/write/resize/kill/pause/resume/ack.
• Route terminal panels, CLI panels, run commands, and legacy terminal sessions through ptyHost when enabled, with legacy fallback when disabled or unavailable.
• Preserve main-side terminal semantics: DEC 2026 filtering, scrollback persistence, alt-screen/idle tracking, and flow-control bookkeeping.
• Fix ptyHost MessagePort acks so main-side backpressure resumes correctly.
• Add host-ready handshake and bounded restart behavior with a 60s stability reset window.
• Preserve manager maps across ptyHost restart so respawn can snapshot stale handles and reinitialize live panels.
• Keep the tester-facing settings checkbox and clarify restart behavior: existing terminals keep their backend; new terminals after restart use the selected PTY mode.
• Add focused tests for the shared ptyHost flow-control state machine.

Follow-up

Tracked in #164:

• Packaged macOS validation.
• Windows/WSL validation.
• Supervisor restart integration coverage.
• Heavy-output and TUI regression validation.
• Decision/work for true direct ptyHost-to-renderer byte delivery.
• Cleanup of duplicated PTY handle shims after behavior stabilizes.

Validation

• pnpm --filter main exec vitest run src/ptyHost/flowControl.test.ts
• pnpm typecheck
• pnpm lint
• Pre-commit hook reran typecheck and lint successfully.

Notes:

• Local environment prints the existing engine warning: repo expects Node >=22.14.0; current shell is Node v20.19.3.
• pnpm lint passes with existing warnings.
• ptyHost remains experimental and default-off unless enabled by config or PANE_USE_PTY_HOST=1.