feat: support ACP session fork

[Leeeon233]

What changed

• advertise sessionCapabilities.fork during ACP initialization
• register ACP session/fork and map it directly to Codex App Server thread/fork
• carry cwd, MCP servers, and additional directories into the forked Codex thread
• install the returned child through the same session-open lifecycle used by new/resumed sessions, including model/mode/config state and stale-open cleanup
• publish available commands for the child session
• document the direct-fork/no-replay invariant

Why

Codex App Server already supports thread/fork, but the ACP adapter did not expose that primitive. ACP clients therefore could not discover or invoke native Codex session forking.

This keeps fork as a lifecycle operation and does not emulate it by replaying history.

Refs agentclientprotocol#315.

Impact

ACP clients can capability-gate a fork action, create an independent child session, and continue prompting it using the existing prompt/resume/load/close/delete paths. The source session remains unchanged.

Validation

• npm run typecheck
• npm test — 324 passed, 29 skipped
• npm run build
• npm pack --dry-run
• npx -p bun@1.3.11 npm run bundle:all — all six CI targets passed
• added unit coverage for capability advertisement, request/config mapping, child state installation, no history replay, and cleanup after post-fork setup failure
• added an opt-in real Codex E2E that verifies inherited history and continued prompting; it was not run locally because this environment has neither CODEX_API_KEY nor OPENAI_API_KEY