chore(ship): v0.15.0 — VERSION, CHANGELOG, README, upgrading-agents, CLAUDE.md

Bumps VERSION → 0.15.0 and package.json → 0.15.0 (resolves the pre-existing
drift — on master, VERSION=0.14.0 but package.json=0.13.1; src/version.ts
reads package.json, so this is what the binary prints now).

CHANGELOG lands the release-summary entry in the GStack voice + the full
itemized change list (11 new modules, 3 new tables, queue correctness
fixes, trust-model additions, 159 new unit tests). Voice rules respected
— no em dashes, no AI vocabulary, real file names + real numbers.

README gets a "Durable agents: `gbrain agent` (v0.15)" section next to
the Minions block, with the three canonical CLI shapes (single run,
fanout-manifest, logs --follow) and a pointer to plugin-authors.md.

docs/UPGRADING_DOWNSTREAM_AGENTS.md gets a full v0.15.0 section covering
the four adoption steps downstream agents (Wintermute and similar) need:
(1) worker opt-in via GBRAIN_ALLOW_LLM_JOBS, (2) moving custom subagent
defs to a plugin repo, (3) replacing ephemeral subagent runs with durable
`gbrain agent run`, (4) the put_page namespace rule for agent-driven writes.

CLAUDE.md updated with concise per-file descriptions for every new module:
the handler, aggregator, audit, rate-leases, wait-for-completion,
transcript, plugin-loader, brain-allowlist, tool-defs extraction, agent
CLI + logs CLI, and the registerBuiltinHandlers wiring for subagent
handlers + plugin-loader.

Verified: binary builds (940 modules, 89ms compile), prints `gbrain 0.15.0`,
`gbrain agent --help` shows the new subcommand shape. 170 new tests pass
(full v0.15 surface). Full unit suite passes bar one parallel-load
flake on a pre-existing E2E (graph-quality, passes in isolation).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/UPGRADING_DOWNSTREAM_AGENTS.md

@@ -358,6 +358,105 @@ upcoming `gbrain crontab-to-minions <file>` helper is P1 in TODOS.
 
 ---
 
+## v0.15.0: durable agent runtime
+
+v0.15 ships `gbrain agent run` / `gbrain agent logs`, a new `subagent` handler
+type in Minions, and a plugin contract for host-repo subagent defs. None of the
+existing skills need surgery. The question for downstream agents is *how* to
+adopt the new runtime, not how to patch around a breaking change.
+
+### 1. Opt in on the worker
+
+The subagent handlers (`subagent` and `subagent_aggregator`) are protected names
+and the worker registers them only when explicitly enabled. Treat this as a
+cost decision — the handler calls the Anthropic API, so flip the flag only on
+workers you want paying those bills.
+
+```bash
+GBRAIN_ALLOW_LLM_JOBS=1 ANTHROPIC_API_KEY=sk-ant-... gbrain jobs work
+```
+
+Worker startup prints one of:
+
+```
+[minion worker] subagent handlers enabled (GBRAIN_ALLOW_LLM_JOBS=1)
+[minion worker] subagent handlers disabled (set GBRAIN_ALLOW_LLM_JOBS=1 to enable)
+```
+
+### 2. Ship your subagents as a plugin (Wintermute + similar)
+
+Move your custom subagent definitions out of your gbrain fork and into your own
+repo as a plugin. Concretely:
+
+```
+~/<your-agent>/gbrain-plugin/
+├── gbrain.plugin.json
+└── subagents/
+    ├── meeting-ingestion.md
+    ├── signal-detector.md
+    └── daily-task-prep.md
+```
+
+`gbrain.plugin.json`:
+
+```json
+{
+  "name": "wintermute",
+  "version": "2026.4.20",
+  "plugin_version": "gbrain-plugin-v1"
+}
+```
+
+Each `subagents/*.md` is a plain-text agent definition — YAML frontmatter +
+body-as-system-prompt. Recognized frontmatter fields: `name`, `model`,
+`max_turns`, `allowed_tools` (must subset the derived brain-tool registry).
+
+Turn it on:
+
+```bash
+export GBRAIN_PLUGIN_PATH="$HOME/<your-agent>/gbrain-plugin"
+```
+
+Worker startup prints `[plugin-loader] loaded '<name>' v<ver> (N subagents)`
+per plugin; any rejection (bad manifest, unknown tool in `allowed_tools`,
+version mismatch) shows up as a loud warning at startup, not a silent dispatch-
+time failure. See `docs/guides/plugin-authors.md` for the full contract.
+
+### 3. Replace ephemeral subagent runs with durable ones
+
+If your agent currently spawns ephemeral subagents (OpenClaw `Agent()`, ad-hoc
+Anthropic API calls, etc.) for work that should survive crashes, sleeps, or
+worker restarts, migrate those to `gbrain agent run`. The durability is free:
+
+```bash
+gbrain agent run "analyze my last 50 journal pages for recurring themes" \
+  --subagent-def analyzer --fanout-manifest manifests/journal-pages.json
+```
+
+Every turn persists to `subagent_messages`, every tool call is a two-phase
+ledger, and `gbrain agent logs <job>` shows where it died + what the last
+successful call returned. No more "re-run from scratch because the session
+context evaporated."
+
+### 4. `put_page` from subagents writes under an agent namespace
+
+If you adopted the v0.15 subagent runtime, note that `put_page` calls
+originating from a subagent's tool dispatch MUST target
+`wiki/agents/<subagent_id>/...`. The schema shown to the model enforces this
+on first try; a server-side fail-closed check rejects anything else. This
+does NOT affect your skill files, CLI put_page calls, or MCP put_page —
+only tool-dispatched writes from inside an LLM loop.
+
+Aggregation output (the final "here's what all N children found" brain page)
+goes via a separate trusted CLI path, not through a subagent tool call, so
+it can write anywhere you want.
+
+Iron rule: **never grant an agent write access beyond its namespace**. The
+server-side check exists because dispatcher bugs happen; treat it as defense
+in depth, not the primary boundary.
+
+---
+
 ## Future versions
 
 When gbrain ships a new version, this doc will be updated with the diffs for that