docs sync + revert auto-unfree removal

revert the earlier 'operator must set allowUnfree' move:
per-agent containers evaluate their own nixpkgs and the operator's
host-level allowUnfree doesn't propagate in. restoring the scoped
allowUnfreePredicate inside both the claude-unstable overlay and
harness-base.nix; documented in README + gotchas as 'nothing to
set on the operator side'.

docs:
- claude.md file map adds crash_watch.rs, kick_agent on coordinator,
  /api/model + journald viewer + bind-with-retry references.
- scratchpad rewritten to reflect the recent run.
- web-ui.md: notification row + browser notifications section,
  state row (badge + model chip + last-turn chip + cancel button),
  per-agent inbox, /model slash, /cancel-question + journald
  endpoints, focus-preservation on refresh.
- turn-loop.md: --model is read from Bus::model() per turn (runtime
  override via /model); recv(wait_seconds) up to 180s with the
  rationale; ask_operator gains ttl_seconds; new TurnState section;
  kick_agent inbox-on-startup hint.
- approvals.md: ttl/cancel resolution paths for operator questions.
- persistence.md: /state/hyperhive-model file.
- gotchas.md: web UI port collision policy (rename, don't probe);
  bind retry + SO_REUSEADDR shape; auto-unfree restored.
- todo.md: cleaned up empty sections and stale entries; /model
  shipped, dropped from the list.

docs/persistence.md

@@ -46,6 +46,15 @@ setups). On open failure the `Bus` falls back to no-store mode
 rather than crashing the harness — events still broadcast over SSE,
 just nothing persisted.
 
+### `/state/hyperhive-model` (per agent)
+
+Single-line text file holding the claude model name currently
+selected for this agent (default `haiku` when absent). Written by
+`Bus::set_model` whenever the operator flips it via `/model
+<name>` in the web terminal. Read once at harness boot in
+`Bus::new`. Path overridable via `HYPERHIVE_MODEL_FILE`.
+Survives destroy/recreate, gone on `--purge`.
+
 ## State dirs (per agent)
 
 Under `/var/lib/hyperhive/agents/<name>/`: