Feat v0.8.2 capability negotiation (#69)

Extensions declare environment requirements via capabilities.required and
capabilities.optional in their manifest. The kernel validates at install
time — required capabilities reject with HTTP 422 and rollback, optional
capabilities log a warning. Runtime query via settings.has_capability().
Admin endpoint GET /api/v1/admin/capabilities re-probes live.

Detected capabilities: postgres, pgvector, object_storage, s3, workspace.

18 new tests, all passing with -race.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,46 @@
 
 All notable changes to Armature are documented here.
 
+## v0.8.2 — Capability Negotiation
+
+Extensions declare environment requirements in their manifest. The kernel
+validates at install time and exposes a runtime query for graceful degradation.
+
+**Manifest: `capabilities` block**
+
+- `capabilities.required` — array of capability names. Install is rejected
+  (HTTP 422) if any are unavailable. Rollback deletes the package row.
+- `capabilities.optional` — array of capability names. Install succeeds
+  regardless; extensions query at runtime via `settings.has_capability()`.
+
+**Detected capabilities:** `postgres`, `pgvector`, `object_storage`, `s3`,
+`workspace`.
+
+**Starlark API**
+
+- `settings.has_capability(name)` — returns `True` or `False`. Always
+  available (no permission required).
+
+**Admin API**
+
+- `GET /api/v1/admin/capabilities` — re-probes and returns current state.
+
+**Internal**
+
+- New: `handlers/capabilities.go` (detection, parsing, validation, admin handler).
+- New: `handlers/capabilities_test.go` (14 tests).
+- New: `sandbox/settings_module_test.go` (4 tests).
+- Modified: `handlers/packages.go` — replaced stub `checkCapabilities` with
+  real validation; `SetCapabilities` setter.
+- Modified: `handlers/packages_bundled.go` — bundled packages with unmet
+  required capabilities are skipped on startup.
+- Modified: `sandbox/settings_module.go` — `has_capability` builtin.
+- Modified: `sandbox/runner.go` — `capabilities` field + `SetCapabilities` setter.
+- Modified: `server/main.go` — `DetectCapabilities` at startup, wired to
+  runner and package handler, admin route registered.
+
+---
+
 ## v0.8.1 — Workspace Module
 
 New `workspace` sandbox module. Managed disk directories for extensions