docs(hyperguild): document brain pass-rate subcommand and /pass-rate endpoint

Adds pass-rate to the CLI README's subcommand block. Updates CLAUDE.md
to note the new /pass-rate endpoint alongside the existing brain
HTTP REST API surface. Updates the session_log MCP tool's
final_status description to reflect the new pass|fail|skip vocabulary
introduced by Plan 5's SKILL.md instrumentation; the aggregator
still accepts legacy ok|error|skipped values for backwards compat.

cmd/hyperguild/README.md

@@ -69,6 +69,35 @@ EOF
 knowledge/example-lesson.md
 ```
 
+### `hyperguild brain pass-rate <skill>`
+
+Returns the pass rate for a skill over a lookback window. Computed
+on-demand from `brain/sessions/*.jsonl`.
+
+```bash
+$ hyperguild brain pass-rate tdd
+tdd: 47 / 50 = 94% (window: 7d)
+
+$ hyperguild brain pass-rate tdd --window 30d --json
+{
+  "skill": "tdd",
+  "window": "30d",
+  "pass": 142,
+  "fail": 8,
+  "skip": 5,
+  "total": 155,
+  "pass_rate": 0.9467
+}
+```
+
+Flags:
+
+- `--window` — lookback window (default `7d`; accepts `Nh`, `Nd`)
+- `--json` — emit the raw response envelope
+
+Skills with no logged invocations return zero counts and `pass_rate: null`
+(indicating "no data", distinct from "always passes").
+
 ### `hyperguild mode <cloud|client-local|sovereign>`
 
 Writes a `.mcp.json` template for the chosen operating mode.