[codex] Fix Nexus session handoff routing

[windoliver]

Summary

Fixes the Nexus-backed session handoff path so Codex work can be routed to Claude reviewers and reflected accurately in session state and the TUI.

What Changed

• Advertise and implement ACP client filesystem and terminal capabilities for spawned agents.
• Poll the Nexus session-scoped contribution store during grove session start when Nexus is configured.
• Resolve the Nexus zone from the persisted Grove namespace before falling back to GROVE_ZONE_ID or default.
• Link accepted routed contributions into both local and Nexus session ledgers.
• Mirror terminal session completion back to Nexus so local and Nexus session status stay aligned.
• Tag MCP-written contributions against the active session in Nexus and local modes.

Root Cause

The headless session orchestrator mirrored the session to Nexus, but still polled local SQLite for contributions. Agent MCP servers wrote session-scoped contributions to Nexus, so downstream reviewers did not reliably receive handoffs. The session count and completion mirrors also had separate gaps, which left the TUI showing 0c or Nexus sessions still marked active after local completion.

Validation

• bun --config=/dev/null test src/core/session-orchestrator.test.ts src/cli/commands/session.test.ts --timeout 30000
• bun --config=/dev/null test src/core/acp-runtime.spawn.test.ts --timeout 30000
• bun run typecheck
• bun run check (passes with existing warnings)
• timeout 180s bun run build
• git diff --check
• Live tmux Nexus validation: Codex created and committed nexus-hello-final.txt, Claude reviewed the actual coder workspace, emitted [DONE], and both local and Nexus session stores reported completed, achieved, contributionCount: 3. TUI capture showed the completed row as 3c · achieved.