feat(telemetry): complete bb-buff.1.3 - Backend Liveness Refactor

STORY:
The session backend needed to aggregate agent health from a live
telemetry stream rather than static bead metadata. This refactor
makes liveness signals real-time and accurate.

COLLABORATION:
We extended the ActivityEvent model with a native 'heartbeat' kind,
updated extendActivityLease() to emit through the activity bus, and
refactored getAgentLivenessMap() to prioritize heartbeat activity
history over stale bead metadata.

DELIVERABLES:
- ActivityEvent extended with 'heartbeat' kind
- extendActivityLease() emits heartbeats through activity bus
- getAgentLivenessMap() prefers telemetry over static metadata
- Registry APIs support projectRoot injection for testing
- Tests verify preference logic via TDD

VERIFICATION:
- 93/93 tests PASSING
- Heartbeat override verified in isolated temp projects

CLOSES: bb-buff.1.3
BLOCKS: bb-buff.3.2, bb-buff.3.3, bb-buff.2.1

skills/beadboard-driver/references/command-matrix.md

@@ -1,23 +1,21 @@
 # Command Matrix
 
-## Preflight and Identity
+## Bootstrapping and Handshake
 
-- `node skills/beadboard-driver/scripts/resolve-bb.mjs`
-  - Output: `{ ok, source, resolved_path, reason, remediation }`
-- `node skills/beadboard-driver/scripts/session-preflight.mjs`
-  - Output: `{ ok, tools.bd, bb }` or `{ ok:false, error_code }`
-- `node skills/beadboard-driver/scripts/generate-agent-name.mjs`
-  - Output: `{ ok, agent_name, attempts, collisions }` or `{ ok:false, error_code }`
-- `node skills/beadboard-driver/scripts/readiness-report.mjs --checks <json> --artifacts <json>`
-  - Output: `{ ok, checks, artifacts, dependency_sanity, summary }`
+- `node scripts/bb-init.mjs --register <name> --role <role> --json`
+  - Output: `{ ok, agent_id, mode, lease, timestamp }`
+- `node scripts/bb-init.mjs --adopt <id> [--non-interactive] --json`
+  - Output: `{ ok, agent_id, mode, lease, timestamp }` or `{ ok:false, error }`
 
 ## Coordination Commands (`bb`)
 
 - `bb agent register --name <agent> --role <role>`
-- `bb agent list`
+- `bb agent activity-lease --agent <agent> [--json]`
+  - Output: `{ ok, command, data: AgentRecord }`
+- `bb agent list [--role <role>] [--status <status>]`
 - `bb agent show --agent <agent>`
 - `bb agent send --from <agent> --to <agent> --bead <id> --category <HANDOFF|BLOCKED|DECISION|INFO> --subject <text> --body <text>`
-- `bb agent inbox --agent <agent> [--state unread|read|acked]`
+- `bb agent inbox --agent <agent> [--state unread|read|acked] [--bead <id>]`
 - `bb agent read --agent <agent> --message <message-id>`
 - `bb agent ack --agent <agent> --message <message-id>`
 - `bb agent reserve --agent <agent> --scope <path> --bead <id> [--ttl <minutes>] [--takeover-stale]`
@@ -31,3 +29,10 @@
 - `bd update <bead-id> --status in_progress --claim`
 - `bd update <bead-id> --notes "<evidence>"`
 - `bd close <bead-id> --reason "<summary>"`
+
+## Legacy/Internal Scripts
+
+- `node skills/beadboard-driver/scripts/resolve-bb.mjs`
+- `node skills/beadboard-driver/scripts/session-preflight.mjs`
+- `node skills/beadboard-driver/scripts/generate-agent-name.mjs`
+- `node skills/beadboard-driver/scripts/readiness-report.mjs --checks <json> --artifacts <json>`