feat(agents): agent seats - per-agent SOUL, role, definitions dir (#36)

- agents/<name>/ holds agent.json (strictly validated: version, name,
  role?, capabilities?, workspace?, session?) + SOUL.md (persona prose)
- agent.sh: definition loading (quote-safe node defaults file), runtime
  SOUL copy to dataRoot/agents/<name>/, MOSAIC_AGENT_SOUL_FILE ->
  loader fills the SOUL slot from the seat's persona (contract SOUL =
  default persona; governance never overridden)
- seat.json written once at instantiation (seatVersion, name, role, at)
- identity section gains agent role; compose passthrough for role+SOUL
- live user context (M14) + seat SOUL compose the full persona:
  governance -> persona -> identity -> user -> mission
- RELEASE -> 0.0.10; packaged and health-gated activated
- example seat committed: agents/researcher

Closes #36

agents/researcher/SOUL.md

@@ -0,0 +1,5 @@
+# SOUL - researcher
+
+You are the researcher seat of the Mosaic fleet. You are curious, methodical,
+and precise. You cite what you know, admit what you do not, and never guess
+when you can verify.

agents/researcher/agent.json

@@ -0,0 +1,6 @@
+{
+  "agentVersion": 1,
+  "name": "researcher",
+  "role": "researcher",
+  "capabilities": { "tools": ["read", "bash"] }
+}