fix(session_start): report which pin a re-pin moved (#517)

[atlas-from-plumb]

Why

After a re-pin, session_start printed a neutral Re-pinned: <from> → <to>. It could not say which pin had moved: the caller's own per-agent pin, or the connection's pin, which every agent without a pin of its own follows. The daemon makes that choice, but the tool only got the previous root back, and it computed "from" from the caller's own root. That gave two wrong outputs:

• Anonymous default-scope re-pin on a shared connection. It moves the connection's pin, and the seeded peer shards follow it. The caller was not told that other agents moved too.
• scope: "connection" from an agent that has its own pin. The agent's own root was shown as "from", and the # Workspace: header showed the connection's new root. The agent itself still resolves against its own pin, so the header named a root its relative paths do not use.

Change

• The WithRepin callback now returns a tools.RepinReport instead of a bare root. The report holds:
  • Root: the resolved root;
  • Scope: agent or connection;
  • From: that pin's previous root;
  • Followers: how many other agents' shards followed a connection move;
  • Effective: the root the caller's next relative-path call resolves against, read through workspaceFor under the caller's own ctx after the move.
• Daemon side (internal/cli):
  • repinWorkspace is the callback and returns the report.
  • repinWorkspaceFrom and repinConnection return a repinOutcome.
  • repinAgent also returns the shard's previous root, read under the same sh.mu lock that moves it.
  • followConnectionShards returns the ids it moved.
  • repinReport (new conn_repin_report.go) leaves the caller out of the follower count.
• Tool side: the full and brief packets both print one of two headlines, followed by a next-call line:
  • Re-pinned your pin: A → B
  • Re-pinned this connection's pin: A → B (N other agents follow it)
  • Next relative-path call resolves against: <root>. When that root is the agent's own pin rather than the connection's new root, the line adds (your own pin, not the connection's).
• # Workspace: (and the rest of the packet) now uses the caller's own root.
• Every variant starts with Re-pinned, so the existing absence checks on that bare prefix still apply.
• The brief-packet budget test (TestSessionStartBrief_ByteBudget) and the brief re-pin case (1536 bytes) still pass.

Tests

New tests in internal/cli/session_start_repin_report_test.go. They run through the real tools.SessionStart wiring (newSessionStartTool), and each one runs with both detail: full and detail: brief:

• identified agent-scope re-pin on a shared connection: reports your pin, and the connection pin is unchanged;
• anonymous default-scope re-pin on a shared connection: reports the connection's pin with 2 other agents follow it, and both peers really moved;
• scope: "connection" from an agent with its own pin: reports the connection's previous root, the header and next-call line name the agent's own root, and the peer followed;
• scope: "connection" from a seeded agent: the caller follows too but is not counted;
• single-agent connection: no other agent follows it, with a same-root control that prints no Re-pinned at all.

TestRepinAnnouncement is now table-driven and covers every variant. The existing stubs and assertions are updated to the new signature and wording.

Evidence:

• Main vs this branch: all 5 new cli tests fail on origin/main (b5a4392) with the new test file copied in. They fail on the wording, on the wrong "from" and on the wrong header. They pass here.
• Mutation checks: 11 of 11 mutants killed, including:
  • counting the caller as a follower;
  • Effective falling back to Root;
  • followConnectionShards reporting nothing;
  • labelling an agent move as a connection move;
  • taking "from" from the caller's root;
  • dropping the same-root suppression;
  • using the requested root as the header;
  • dropping the own-pin suffix;
  • dropping the block from the brief packet, and from the full packet.
• go test ./... -count=1 exits 0.
• go test -tags=integration ./internal/cli/ -count=1 exits 0.
• golangci-lint is clean.
• check-changelog-placement --require-base is OK.

Closes #517.

🤖 Generated with Claude Code

[golimpio]

Approved: two review rounds, both blockers fixed and confirmed (stress guard red on prior head); green CI on 604745a.

[golimpio]

Taken over to merge at the owner's request. Its own two review rounds are recorded on the PR, with both blockers fixed and confirmed; updated with main by a clean merge.