Implement machine placement and leases

[andrei-hasna]

Summary

• add first-class loop machine placement for exact machines, single-run pools, and explicit fanout targets
• advertise runner hostname, labels, and capabilities through runner register/claim heartbeats
• add fanout-aware run keys, leases, SQLite/Postgres schema support, API claim/advance behavior, CLI/MCP inputs, exports, and docs

Validation

• bun run typecheck
• bun run test:boundary
• bun run build
• HOME="$PWD/.test-home" XDG_CONFIG_HOME="$PWD/.test-home/.config" bun test (545 pass)
• staged secrets scan passed before commit; committed diff secrets scan passed before push

Notes

• plain bun test with the real HOME hits host-level config write restrictions in this sandbox; the full suite passes with a writable test HOME.
• local git hooks printed "Can't find lefthook in PATH" during commit/push but did not block either operation.