feat(local_agents): tier-3 index + picker merge (v0.8.0)

Worldtree's GET /agents doesn't return consumer-defined (tier-3)
agents — the public list excludes them by design. Confirmed live in
v0.7.0's smoke. Without server-side knowledge, ratatoskr's picker
couldn't show tier-3 agents the operator had defined; the workflow
was "remember the agent_id, pass --agent ratatoskr:<name>
explicitly." Friction grows with every tier-3 agent.

## Fix: client-side index, merged at picker time

New module `ratatoskr.local_agents` maintains a JSON-backed index at
$XDG_CONFIG_HOME/ratatoskr/local_agents.json (override via
$RATATOSKR_LOCAL_AGENTS). `tier3` CLI define / patch / delete update
the index as side-effects. `tui._resolve_then_run` loads the index
after `list_agents(client)` and appends entries not already in the
remote list (dedup by agent_id; remote wins on conflict).

Library-level `tier3.define_agent` / `patch_agent` / `delete_agent`
stay pure — local persistence lives in the CLI layer (`_run_define`
etc.), not in the library functions. Tests of the library don't
touch the filesystem.

## Public surface

  ratatoskr.local_agents:
    LocalAgentEntry (frozen dataclass)
    load_local_agents() -> list[LocalAgentEntry]
    add_local_agent(entry)
    update_local_agent(entry)  # same semantics as add (agent_id key)
    remove_local_agent(agent_id)
    make_description(system_prompt) -> str  # synthetic picker label

Failure modes are lenient: missing file → empty index; corrupt JSON
or schema mismatch → empty index (no crash). The picker continues
to show foundational agents either way; tier-3 surface degrades to
the pre-v0.8.0 workflow.

## Picker integration

Local entries convert to ratatoskr.sessions.AgentInfo with synthetic
fields:
  name        = agent_name (from LocalAgentEntry)
  description = "(tier 3) <first non-empty line of system prompt>"
  version, capabilities, supported_models, persona_traits, ui_hints
    = None / [] / [] / {} / {}

If Worldtree later starts returning tier-3 in GET /agents, this
module's role narrows to redundant local cache; can be removed
cleanly since the dedup-by-agent-id keeps remote-wins behavior.

## Tests

286/286 GREEN (was 265, +21: 20 local_agents + 1 picker-merge
integration). Ruff clean. Tests isolate the index via
$RATATOSKR_LOCAL_AGENTS pointed at pytest's tmp_path — no pollution
of operator's real ~/.config/ratatoskr/.

## Manual smoke

Sindra-like define against personal Worldtree:
  python -m ratatoskr.tier3 define --name foo --system-prompt "..." --model X
  cat ~/.config/ratatoskr/local_agents.json
  # ratatoskr --new picker now shows ratatoskr:foo alongside mimir et al.

Cross-machine: the file is per-host. Operator can sync via dotfiles
if needed; out of scope for this commit.

Minor bump (v0.7.1 → v0.8.0) — new public module + new picker
behavior (more agents shown). No caller-side breaking changes.

persistent-memory.md

@@ -32,9 +32,9 @@ separate dev team rather than an in-tree Worldtree tool.
 
 ## Current state / in-flight
 
-_As of 2026-05-25 (post-v0.7.1 thinking coalesce-by-newline):_
+_As of 2026-05-25 (post-v0.8.0 local tier-3 agent index in picker):_
 
-**Status: v0.7.1 shipped.** Ten core features complete (`sse_client`
+**Status: v0.8.0 shipped.** Eleven core features complete (`sse_client`
 #1, `sessions` #2, `cli` #3, `tui` #4, `--end-user-id` #5, TUI
 startup error visibility #6, presenter contract semantics amendment
 #12, startup agent picker #8, §5 layout reshape + Tools pane #13)
@@ -51,7 +51,8 @@ Static in the footer (static "Tools" v1; dynamic when more tabs
 land). CLI mode (--send) unaffected by design — INV-018.
 
 Last commits on `main`:
-- v0.7.1 fix(tui): coalesce thinking deltas on `\n` — no more per-token newlines
+- v0.8.0 feat(local_agents): JSON-backed local tier-3 index + picker merge
+- `9918c10` fix(tui): coalesce thinking deltas on `\n` (v0.7.1)
 - `c086ae2` feat(tier3): ratatoskr.tier3 module + CLI (v0.7.0)
 - `d356990` refactor(tui): thinking streams into thinking-log (v0.6.5)
 - `82437bd` style(tui): picker highlighted item → Aurora blue (v0.6.4)