feat(webui): system-health dashboard (Closes #639)

Phase 1 child of the Web Console epic #631. Adds the operator-facing
system-health dashboard on top of the read-only system-health API landed
by #634, so runtime problems are visible on a surface instead of being
discovered late through failed LLM sessions.

- webui/system_health_views.py (new): renders the SystemHealthSnapshot as
  readiness, stale-runtime parity, version/uptime, dependency, MCP
  namespace, probe-error, and recovery cards.
- webui/app.py: GET /system-health, sharing load_system_health() with the
  JSON API so page and API cannot disagree. ?deep=1 behaves as on the API.
- webui/layout.py: nav entry and health card/badge styles.
- tests/test_webui_system_health_dashboard.py (new, 26 cases).
- docs/webui-local-dev.md: route, field authority, and redaction split.

Readiness honesty is preserved from the API: ready and readiness_complete
render separately, a probe that did not run is listed under "Not probed"
rather than counted healthy, and mutation safety is never claimed when the
runtime is stale or parity is indeterminate.

Redaction is split by field kind. Free text (probe details, reasons, probe
errors) passes through system_health.redact. Structured fields (commit
SHAs, probe names, statuses, timestamps) are HTML-escaped only: redact's
opaque-token rule matches any run of 32 or more characters, so routing a
40-character git SHA through it rendered "[redacted]" and blanked the
parity evidence the page exists to show.

Non-goals honored: no restart or reload controls (Phase 2, #642), no
manual process-kill guidance (#630). Read-only throughout.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/webui-local-dev.md

@@ -54,6 +54,7 @@ status, onboarding checklist state, and the fail-closed error payloads (#635).
 | `/` | Home / operator overview |
 | `/health` | JSON liveness (`status`, `service`, `mode`, `timestamp`, `uptime_seconds`) |
 | `/api/v1/system/health` | Structured read-only system health (#634) |
+| `/system-health` | System-health dashboard — readiness, version/uptime, dependencies, MCP namespaces, stale-runtime parity (#639) |
 | `/queue` | Live PR and issue queue dashboard (#429) |
 | `/api/queue` | JSON queue export with pagination metadata |
 | `/projects` | Project registry list with status and onboarding progress (#427, #635) |
@@ -233,6 +234,37 @@ health, workflow/schema SHA-256 hashes, and stale-runtime warnings when the
 checkout is behind merged safety-gate changes. Restart guidance links to #420;
 no tokens or MCP restart actions are exposed.
 
+## System-health dashboard (#639)
+
+`/system-health` renders the same snapshot the `/api/v1/system/health` API
+returns, so the page and the API can never disagree. Cards: overall readiness,
+stale-runtime parity, version and uptime, dependency probes, MCP namespaces,
+probe errors (only when present), and recovery pointers. `?deep=1` opts into
+the network probe exactly as the API does; the plain page load stays cheap.
+
+Field authority and honesty rules:
+
+* `ready` and `readiness_complete` are shown separately. A snapshot whose
+  required probes never ran is not the same as one that ran them and passed,
+  and the page never collapses the two into an unproven green.
+* A probe that did not run appears under **Not probed**, never as healthy.
+* `stale_runtime.mutation_safe` is displayed verbatim from the API. When the
+  runtime is stale, or when parity is indeterminate, the page warns and does
+  not claim mutation safety.
+* MCP namespaces are reported `unproven`: the web process runs outside the
+  IDE-managed MCP client and cannot prove that path (#543).
+
+Redaction is split by field kind. Free text — probe details, readiness and
+parity reasons, probe errors — passes through `system_health.redact`.
+Structured fields — commit SHAs, probe names, statuses, timestamps — are
+HTML-escaped only, because `redact`'s opaque-token rule matches any run of 32
+or more characters and would otherwise blank every 40-character git SHA, which
+is precisely the evidence the parity view exists to show.
+
+The dashboard is read-only: no restart, reload, or process-kill control. Those
+arrive in Phase 2 (#642). Recovery guidance points at the sanctioned client
+reconnect / operator restart path — never a manual daemon kill (#630).
+
 ## Deployment boundary (#435)
 
 MVP serves on loopback by default. Binding `0.0.0.0` or `::` is **refused**