Change-notification ping for moved official lines (transport for the worktree_status freshness payload)

[Foxfire1st]

Context (follow-up from #54)

The #54 series made base-currency checkable at three points: lifecycle start (context_packet include_freshness), worktree start (stale-base preflight), and mid-task (worktree_status freshness + worktree_sync). Detection mid-task is still pull-based: an agent only notices the official line moved when it polls worktree_status.

Proposal

A push-style ping that notifies an active session when the official line moves (parallel PR merge fast-forwards code main, carryover advances memory main). The message already exists: the fetch-free freshness block worktree_status computes (baseBehindSource per side + syncHint recommending worktree_sync). Only the transport is missing.

Candidate transports

1. Reuse the worktree_start: ~6 min of silence during provider setup is indistinguishable from a hang #53 background-setup plumbing: a lightweight watcher (or the existing provider watcher loop) writes a freshness-ping.json under the worktree group when source tips move; harness hooks can surface it.
2. MCP notifications/progress or a server-initiated notification once the worktree_start: ~6 min of silence during provider setup is indistinguishable from a hang #53 async/notification machinery settles.
3. Coordinator-level: the next context_packet/worktree_status call flags it (status quo, pull-only baseline).

Designed in #54 sub-task D so the payload shape is stable regardless of transport choice.

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