Add opt-in cross-repo agent messaging (Agents settings toggle)

[niiyeboah]

Summary

Agent-to-agent messaging (dispatch_send_message and list_agents) scopes the addressable peer set to the sender's git repo root — an agent in one repo can't message or even see an agent in another. That's the right default, but it blocks local multi-repo workflows (e.g. an agent in neo coordinating with an agent in brane-uis).

This adds an opt-in toggle that lifts the scoping. When enabled, senderRepoRoot is no longer required and every other agent is addressable. Default behavior is unchanged (off).

Supersedes the original DISPATCH_CROSS_REPO_MESSAGING env-var approach. Per review feedback, the control is now a setting in the Agents settings page instead of an environment variable.

How it works

• Server is the source of truth. The gate is a single server-wide setting (cross_repo_messaging_enabled) in the existing settings table, read by the MCP handler for every agent. It is not per-device.
• UI toggle lives in Settings → Agents. It's backed by a jotai atom for reactive state / instant first paint, but the atom is a cached view: the component GETs the value on mount to hydrate it and only POSTs on an explicit user toggle. There is no mount-time write, so opening settings on one device never clobbers the global gate.

Changes

• apps/server/src/cross-repo-messaging-settings.ts (new): accessor module owning the settings key and the "true"/"false" ↔ boolean encoding — isCrossRepoMessagingEnabled(pool) / setCrossRepoMessagingEnabled(pool, bool). Mirrors agent-type-settings.ts, so the encoding lives in exactly one place.
• apps/server/src/server/mcp-handlers.ts: the addressable-peer gate reads the setting via the accessor; the boolean is resolved once per call and threaded into a single addressableAgents() helper shared by both message delivery and agent listing.
• apps/server/src/routes/system.ts: GET/POST /api/v1/app/settings/cross-repo-messaging (mirrors the existing agent-types / IDE settings routes; validates enabled is a boolean).
• apps/web: crossRepoMessagingEnabledAtom + a CrossRepoMessagingSettings component rendered in the Agents settings section. Server-authoritative hydration, role="alert" on errors, and a request-sequence guard against out-of-order responses.
• Removed the env var from .env.example; updated unit tests to drive the setting; added an e2e regression test.

⚠️ Name-based targeting caveat (unchanged from before): when enabled, a name can resolve to a same-named agent in a different repo — use the agent ID (agt_…) to address one unambiguously. This is called out in the toggle's helper text.

Testing

• pnpm run check (tsc, server + web) — clean
• Backend unit tests (mcp-handlers.test.ts) — 73 pass, incl. the gate logic driven by the setting
• Settings e2e suite — 5/5 pass, incl. a new regression test for the toggle
• Manual browser validation: default off; enabling persists to the server; a fresh second browser context hydrates from the server and does not flip the gate just by viewing settings

Review

Reviewed by three persona agents (backend-security, architecture, frontend-ux) — all approved after one round of changes that moved the toggle to a server-authoritative model.

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