docs(spec): 1.0 decisions F (flush) and G (age across downtime)

- F: Store::flush(now) with a no-op default; Node::flush(now) replaces
  Node::store_mut. The caller's now anchors ages (fixes LogStore's stale
  clock marker, F18).
- G: age_ms is documented as a lower bound. Robots are assumed to have a
  trusted wall clock (RTC or GPS); LogStore adds power-off time on open
  from a wall-clock flush marker. Unknown downtime expires held addressed
  messages and floors held broadcasts' age at unknown_downtime_age_ms.
  Hosts set observed_at.
- AGENTS.md: specs and plans change only to record decisions Kyle has
  discussed and agreed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -24,7 +24,10 @@ architecture in the **spec first**:
 - **Implementation plans:** `docs/superpowers/plans/`
 
 The plans argue from the specs. Where a plan is silent, the **spec rules**.
-Do not edit the specs or the plans — they are design history, not source.
+Do not edit the specs or the plans on your own initiative — they are design
+history, not source. Change one only to record something Kyle has discussed
+and agreed, and record it as a dated decision in that document. Anything
+else that needs changing in a spec goes to Kyle first.
 `README.md` is the user-facing overview and should track the code.
 `THREAT_MODEL.md` is the living security assessment: when a change fixes or
 changes a finding, update its status there (§1 and §10) with the commit.