feat(p5): NL → /agents bg intent-gate workflow

[lantisprime]

Summary

Adds a background workflow kind to the intent-gate so users can launch background agents via natural language (e.g., "scout this repo in background" triggers /agents bg scout <task>).

Changes

• intent-gate.ts: background workflow kind + bg-launch GateDecision variants. classifyGateIntent returns bg-launch with agent + task + profile, defaulting agent to "scout".
• bg-args.ts: --profile <name> parsing mirroring the --backend first-token pattern.
• bg-preflight.ts: profileOverride accepted; project trust stashed into manifest so the worker can reconstruct the library.
• bg-state.ts: BgRunManifest.options.profile field added.
• bg-worker.ts: profile library reconstruction (user ~/.pi/agent/profiles + trusted-project .pi/profiles + built-ins) — same sources as session_start. projectTrusted + projectRegistry threaded to runChildAgent so project-source profiles pass the trust gate end-to-end.
• index.ts: gate handler bg-launch branch + __bgLaunchTestHook test seam (mirrors __gateDispatch). TUI confirm shows "Launch in background ${agent}?"; disableContextFiles, session profileLibrary/deliverResult re-attach, profile resolution all preserved.
• Tests: 3 new intent-gate tests (config, classification, confirm flow), --profile parser tests, worker library-reconstruction + project profile trust-wiring tests.
• Example: agents/examples/intent-workflows-with-background.json sample config.

Security

• TUI confirm before bg launch (REQ-SEC-3)
• disableContextFiles set for bg path (REQ-SEC-5)
• Session profileLibrary + deliverResult re-attached (REQ-7)
• Profile passed structurally, not via string interpolation

Test results

• 33/33 intent-gate tests
• 9/9 bg-args tests
• 21+ worker tests
• All cross-extension scripts pass

Review

Reviewed by codex (gpt-5.5 high) via cmux — 4 review rounds: changes-requested (profile dropped → row library reconruction → project-profile trust gate) → approve-with-nits → approve. No blockers, no non-blocking concerns remaining.