P3-10: publish the current page to herdr's sidebar

[dvzrd]

Summary

Inside a herdr pane with its own HERDR_PANE_ID, wiki-reader publishes the page being read to herdr's sidebar as display-only pane metadata, on by default ([herdr] publish = false to opt out, as decided).

• A Publisher worker thread in crates/wiki-reader/src/herdr.rs runs herdr pane report-metadata with a title and a page token. No socket client, no new dependency.
• Debounced 250 ms; TTL 10 minutes renewed every 4; cleared on exit. --seq is Unix time in ms and strictly increasing, so a restarted reader is not ignored as stale.
• Values pass as --title=… / --token=page=…, control characters stripped, text capped at herdr's 80 characters (long paths keep their tail).
• Never reports agent state. A missing binary or failing call never touches navigation; it stops after three failures in a row. Plugin popups (no pane id) never publish.
• The publisher is created only by run, so tests running inside herdr cannot publish to a real pane. App::sync_herdr hands over page changes from the event loop.
• ADR-0022 records this together with the ordinary-pane launcher.

#128 and #129 are merged; this branch is a single commit on top of main.

Verification

• scripts/check.sh passed.
• Unit tests: exact argv, burst collapse, unchanged page not resent, renewal with increasing seq, clear on drop (not when nothing was published), failure isolation without blocking exit, target-pane rules, text cleaning; config parsing; an app-level test that page changes reach the publisher once with the right title and relative path.
• Not yet verified in a real herdr: what the sidebar shows for the title versus the token, clear on exit, TTL expiry, and behaviour in a pane that hosts an agent. Needs the operator pass; no release.

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