v0.21.0: agent reliability overhaul — plan integrity, target validation, observability pipelines, learning loop

P0 — stop the bleeding:
- prevent premature complete_task(success) when goal involves reachability
- validate run targets: block host-only commands (qm/pct/pvesh) on LXC/VM
- bump MCP client timeout 30s→120s to stop 'context deadline exceeded'

P1 — fix the plan system:
- add replaced_reason column to session_plan_steps (migration 030)
- track WHY steps are replaced (wrong_diagnosis/scope_change/superseded/etc)
- force fresh propose_plan on session resume (reopenSession marks old plan)

P2 — cognitive guardrails:
- SOUL.md scope-gate rule: ask before chasing unrelated subsystems
- auto-upsert knowledge entry on every session close

P3 — observability (all were empty/NULL):
- populate agent_activity.token_count from LLM usage (was always NULL)
- populate nomos_plan_executions linking executions to sessions
- write plan_completion_rate metric on task close

P4 — learning loop (all were empty/NULL):
- auto-classify every run call → classifications table (was 0 rows)
- auto-feedback on session close (was 0 rows)

migrations/030_plan_step_replaced_reason.up.sql

@@ -0,0 +1,8 @@
+-- 030_plan_step_replaced_reason.up.sql
+-- Add replaced_reason to session_plan_steps so the agent must explain why
+-- a step was replaced (wrong_diagnosis, scope_change, blocked, superseded,
+-- operator_override) rather than silently replacing entire plans. The column
+-- is also set by bulk-replace operations (proposePlan, setGoal, reopenSession)
+-- for auditability.
+
+ALTER TABLE session_plan_steps ADD COLUMN IF NOT EXISTS replaced_reason TEXT;