fix: scan all kanban boards for active workers

[the24thLetter]

Summary

• Scan every non-archived Kanban board when GET /api/plugins/kanban/workers/active is called without a board parameter.
• Preserve the existing active-worker contract: an open task_runs row with a PID still counts without requiring tasks.current_run_id.
• Deduplicate resolved SQLite paths so HERMES_KANBAN_DB aliases do not report the same worker multiple times.
• Include the board slug on each worker returned by a cross-board scan.

Root cause

The gateway dispatcher enumerates all boards, but the dashboard endpoint resolved an omitted board parameter to only the current board. A worker running on another board was therefore invisible and the endpoint could incorrectly return count=0.

Scope

This is a narrow reimplementation of the worker-visibility fix from #33923 on current main. It intentionally excludes the unrelated ledger, review-gate, scripts, generated assets, PID fallback, and diagnostic changes from that PR.

Validation

Added focused coverage for:

• preserving the existing open-run predicate without requiring tasks.current_run_id
• finding an active worker on a non-current board when board is omitted
• retaining explicit single-board filtering
• deduplicating multiple board slugs that resolve to the same HERMES_KANBAN_DB

The rebased endpoint and test additions pass Python AST syntax validation. Repository CI is awaiting maintainer approval to run for this fork-originated PR.

[teknium1]

Thanks for the focused fix. The premise is present on current main: plugins/kanban/dashboard/plugin_api.py:1417-1462 resolves an omitted board through the active-board path and queries only that connection, while the gateway dispatcher already enumerates every non-archived board in gateway/kanban_watchers.py:1266-1281. The PR aligns the endpoint with that dispatcher behavior, retains the established task_runs predicate, and covers cross-board lookup, explicit board filtering, and shared-DB deduplication.

Automated hermes-sweeper review.