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

compose.yaml

@@ -24,6 +24,8 @@ services:
       # Interactive TUI mode + agent identity (M13, set by scripts/agent.sh)
       MOSAIC_INTERACTIVE: ${MOSAIC_INTERACTIVE:-}
       MOSAIC_AGENT_NAME: ${MOSAIC_AGENT_NAME:-}
+      MOSAIC_AGENT_ROLE: ${MOSAIC_AGENT_ROLE:-}
+      MOSAIC_AGENT_SOUL_FILE: ${MOSAIC_AGENT_SOUL_FILE:-}
       # mock adapter only: verbatim response for deterministic seam tests
       MOSAIC_MOCK_RESPONSE: ${MOSAIC_MOCK_RESPONSE:-}
       # Documented container auth alternative: provider API key via