feat: add machine-readable run interface for herdr-board

[aorumbayev]

Adds hwf runs list --json, hwf runs get <uuid> --json, and hwf run <name> --detach --json so herdr-board can start and observe runs without reading history.db or importing HWF packages.

What changed

• One JSON object per response with schema_version: 1 and stable error codes: invalid_request, history_unavailable, run_not_found, incompatible_history, launch_rejected. Failures exit 1.
• History reads open SQLite read-only. They never create, migrate, or repair the database. A missing history.db is an empty list. A wrong user_version is incompatible_history.
• ListRuns pages with a keyset cursor on (started_at desc, id desc), default 40, max 200. Console and runs browser keep page one at 40.
• Summary gains heartbeat_at and a current_step object. Detail embeds Summary and carries JSON tags.
• Launch payload gains requireHistory. A detached child exits before step 1 when history is unavailable or the claim is rejected, and the parent reports launch_rejected. Picker launches are unchanged.
• --input parse errors no longer echo the raw value.
• Docs: new ## Runs section in docs/reference.md, README command rows.

Acceptance

Covers the twelve acceptance tests from the brief: claim before UUID, list and get show the same state, restart safety, paging past 40 without loss or duplication, cursor stability when a newer run arrives, distinguishable empty, unavailable, incompatible, and missing results, malformed row warning, concurrent reads during a run, no input leak in any output, attached run and picker unchanged, help and docs.

Compatibility

Console and runs browser no longer rebuild an old-schema database on open. The next writer does. Summary.CurrentLabel is gone (never serialized before).

go tool verify passes.