feat(j5): add join_squadron so a native thread can establish its Squadron home

[Jacksondr5]

Closes #129.

Problem

A thread created without a Squadron (the mobile creation gap, #128) has no supported way to become an A2A participant. list_participants and send_message fail closed with A2ASenderNotJoinedError, and the home registrar only runs from creation paths. Thread Merge 2 is stuck in exactly this state.

Fix

• join_squadron MCP tool. Acts on the authenticated calling thread. Takes an explicit squadron_id (never inferred from folder or name) plus optional client_request_id, and returns the committed squadron, participant, thread ids and placement.
• list_squadrons MCP tool. Read-only directory of every Squadron with its project ids, plus the caller's own project id, callable by unregistered threads. The existing HTTP directory needs a session token an agent doesn't have, so this is the supported way to obtain the exact id.
• SquadronJoinService. Composes the existing A2AHomeRegistrationTransaction and ParticipantPlacementTransactionWriter in one transaction under the ledger append permit. The home is resolved inside that permit, so a replay with the same request id and concurrent retries with different ids both commit exactly one participant.joined. Placement is unknown / native_or_unobserved at the Squadron root, with no invented spawner or fork parent.
• Refusals, each naming state and next command: archived or deleted thread, a different existing home (A2AHomeConflictError), retired identity, and a Squadron that doesn't reference exactly the thread's project. A thread already homed in the requested Squadron gets its existing registration back.
• The "no registered home" error messages now point at list_squadrons and join_squadron.
• Docs: verb contract entries in agent-tools.md, the substrate verb list, the A2A README, and a dated amendment to the Squadron ruling "there is no join, no leave, no move" narrowing it to this one repair case. Only j5/ paths and docs/j5/ were touched.

Testing

• SquadronJoinService.test.ts (real in-memory SQLite): fresh join, same-id replay, different-id retry, three concurrent joins, launch-homed thread gaining placement, project mismatch, missing squadron, home conflict, retired identity.
• mcp/joinSquadron.test.ts (real ledger/registrar/placement layers + toolkit dispatch): a previously unregistered caller is refused by list_participants, lists squadrons, is refused on the wrong project, joins, replays with and without a request id, appears as self, and is refused on a move attempt. Archived caller refused before any write.
• Updated handlers.test.ts, registration.test.ts, SendService.test.ts.
• tsgo --noEmit for apps/server clean, targeted vp lint clean, 50 tests passing across 8 J5 files. Repo-wide checks left to CI.

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[Jacksondr5]

Suggested change

	"The Squadron directory for this environment: every Squadron's squadron_id, name, and the project ids it references, plus your own thread's project id so you can see which Squadron can home you. Use it to obtain the exact squadron_id before join_squadron; it works even when you have no Squadron home yet. Read-only.";
	"The Squadron directory for this environment: every Squadron's squadron_id, name, and the project ids it references, plus your own thread's project id so you can see which Squadron can home you. Use it to obtain the exact squadron_id before join_squadron. Read-only.";

[Jacksondr5]

Suggested change

"Join the Squadron the human selected for you when your thread has no Squadron home yet — the state where list_participants and send_message refuse you. Pass the exact squadron_id, taken from list_squadrons or from the human; that Squadron must reference your thread's project. Your thread, conversation, worktree, and running work stay exactly as they are — only your participant registration is added, placed at the Squadron root with no invented spawner. Calling it again for the Squadron you already belong to returns your existing registration; a thread that already has a different home is refused, because this is not a move. Reuse client_request_id to retry safely.";

"Join a Squadron when your thread has no Squadron home yet. Pass the exact squadron_id, taken from list_squadrons; that Squadron must reference your thread's project. Your thread, conversation, worktree, and running work stay exactly as they are. Calling it again for the Squadron you already belong to returns your existing registration. Reuse client_request_id to retry safely. Warning: you cannot switch Squadrons once you're assigned, be sure you're joining the right one.";