feat(context): Fleet Capabilities in startup context (capability plane Phase 1)

[ProtocolWarden]

What

The first consumer of the capability registry. build_resume_prompt now appends a Fleet Capabilities section to the compiled .console/.context, so an operator/agent sees what the fleet can do — owner, target scope, risk, lane — before acting. This turns the registry from correct-but-inert into read.

How (deliberately dependency-free)

Reads PlatformManifest's capabilities.yaml directly via PyYAML — no platform_manifest/repograph import and no RepoGraph graph-compile. An adversarial review of the plan flagged that the obvious load_default_capabilities() path drags in a full RepoGraph compile + a sibling-checkout import assumption that OperatorConsole (not a pip package; only PyYAML+cxrp) cannot satisfy. The flat authoring YAML already carries owner_repo_id / target_scope / risk / routing.preferred_lane as fields, so a direct read needs none of that.

• Located in-repo when anchored at PlatformManifest, else a sibling PlatformManifest checkout.
• Fail-soft: missing / malformed / non-list / empty registry → section omitted, context compilation never blocked.
• Private capabilities (visibility != public) are never surfaced.
• Routing stays descriptive — read-only legibility, no execution wiring.

Rendered output (real registry):

## Fleet Capabilities
**context_lifecycle_protocol**
- Session Garbage Collection — fleet · mutates_fleet · lane: maintenance
**custodian**
- Repo Health Audit — repo_set(visibility=public) · read_only
**operations_center**
- Board Unblock — fleet · mutates_fleet · lane: maintenance

Tests

14 new tests: locator (in-repo + sibling), scope trichotomy (repo/repo_set/fleet), grouping+sort, private-exclusion, fail-soft (missing/malformed/non-list/empty), and integration with build_resume_prompt.

Note: two pre-existing test_pipeline.py failures (run-dir written_at timestamp-resolution flakes) are unrelated to this change — they reproduce on main with this diff reverted.

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