feat(coord): cross-surface session presence + serialize worktree creation

[wshallwshall]

Why

Parallel sessions here were isolated but blind. The SessionStart banner listed worktrees, not live
sessions — so it could not tell you the collision that actually matters: someone editing the shared
primary right now. And no tool on this machine could see a VS Code session at all.

What was established first

The Desktop app's list_sessions MCP tool enumerates an in-memory map of sessions the app itself
spawned. A session launched by the VS Code extension is never entered into it — not filtered out,
never registered — so it is invisible there and unaddressable. Verified against a live VS Code session
sharing the default config root, which rules out the per-login explanation.
<config-root>/sessions/<pid>.json is the only registry carrying every surface.

What this adds

scripts/coord/presence.ps1 — a read-only roster of who is actually live in this repo, across
surfaces. Wired into the SessionStart banner, which now calls out a peer in the shared primary and warns
when a peer is on a surface that session messaging cannot reach.

Liveness is fenced, not a pid check: pids get reused and those records outlive their process, so it
compares each process's real start time against the recorded session start. Claude Code ships a
procStart field for exactly this, but here it serialises as absent and the guard passes
unconditionally — a bare pid check reports a recycled pid as a live session.

scripts/coord/lock.ps1 + new.ps1 — git worktree add -b <name> <base> writes .git/config, so
concurrent adds race .git/config.lock, leaving orphaned branches behind. That call is now serialized
through the same atomic exclusive-create claim.ps1 uses. It retries and never steals: with no
reliable liveness signal on this host, breaking a lock you cannot prove is abandoned re-opens the race
it exists to close, so on timeout it fails loudly and names the holder.

Verification

19 new tests, all driving the real scripts as subprocesses against throwaway repos and fixture
config roots — never a Python copy of the rules.

Two mutation checks, because a green test that would pass anyway proves nothing:

Mutation	Expected	Result
Disable the start-time fence in presence.ps1	the pid-reuse test fails, others hold	✅ fails; dead_pid still passes, so it is surgical
CreateNew → Create in lock.ps1	both concurrency tests fail	✅ both red

The lock's headline test launches eight real concurrent processes and asserts exactly one wins and
the losers fail rather than silently proceed — eight because a read-modify-write in this codebase was
measured silently losing 4 of 8 concurrent PowerShell writes.

The SessionStart tests pin the fail-safe contract: a missing, throwing, or junk-emitting presence.ps1
costs the live-session list and nothing else, because whatever that hook prints is the chat's starting
context. One test asserts the positive arm too — otherwise a no-op integration would satisfy every
absence assertion.

ruff format, ruff check, mypy clean.

Not included

A live cross-session defect in user-global ~/.claude/settings.json (three hooks share one
$env:TEMP\claude-waiting.flag, so any session's prompt-submit clears a flag another live session set)
is out of scope for this repo and was blocked by the permission classifier. The tested fix is in the
session notes for the owner to apply by hand.

🤖 Generated with Claude Code