Add README sessions pulse helper

[quick-ricon]

Goal

Add a shimmer-owned README helper that can refresh the local session pulse snapshot used by README.tsx.

The README wants to show local machine session topology as a deliberate snapshot, not as build-time nondeterminism. The useful numbers are:

• recorded sessions in the local sessions corpus
• live session processes
• branch tips in the session forest

Why tips, not branches

A branch tip is a terminal session head: a session node with no child sessions. That answers the operational question: "how many resumable/current conversation heads exist?"

Counting "branches" is ambiguous and overcounts history: it could mean fork events, root-to-leaf paths, named continuations, or internal split segments. A long linear continuation chain should still count as one tip; a singleton session with no children is also one tip.

Current findings

Read-only probes on Or's machine on 2026-06-23:

sessions query --limit 10000 --sql 'select count(*) from sessions' --format json
# recorded sessions: 659

sessions ps --json
# live session processes: 0

For branch tips, current sessions query does not expose structured session-to-session edges. The only path found today is parsing the fork notice text emitted by sessions copy:

with fork_edges as (
  select
    session_id as child_session_id,
    substr(
      text_excerpt,
      instr(text_excerpt, 'forked from ') + length('forked from '),
      36
    ) as parent_session_id
  from messages
  where text_excerpt like '[Session Fork Notice]%'
    and instr(text_excerpt, 'forked from ') > 0
)
select
  (select count(*) from sessions) as recorded_sessions,
  (select count(*) from fork_edges) as fork_edges,
  (select count(*) from sessions s where not exists (
    select 1 from fork_edges e where e.parent_session_id = s.session_id
  )) as branch_tips;

That returned:

recorded_sessions: 659
fork_edges: 3
branch_tips: 657

Proposed shimmer work

Add a task group such as:

shimmer readme sessions-pulse
# or
shimmer readme:sessions-pulse --json

It should:

1. Use sessions query --limit <large> for recorded session count.
2. Use sessions ps --json for live process count.
3. Count branch tips from a structured edge source if available.
4. Until sessions query exposes session edges, either:
   • use the fork-notice SQL parser with a clear caveat, or
   • block on / coordinate a sessions enhancement that projects sourceSessionId / fork edges into SQL.
5. Print a README.tsx snippet or JSON payload that makes refreshing the manual snapshot easy.
6. Include BATS coverage with a mocked sessions binary; tests must not depend on Or-machine session history.

Acceptance

• README.tsx documents the refresh command in comments.
• README.md includes a manual snapshot with recorded sessions, branch tips, live processes, capture date, and source.
• README generation stays deterministic in CI; no build-time calls to sessions.
• The helper has tests and handles missing sessions / jq with clear errors.