feat: add detached mode capabilities - #617

Description\n\nAdds detached-mode capability reporting and degradation behavior for stateless, read-only, and containerized Headroom deployments.\n\nCloses #180\n\n## Type of Change\n\n- [ ] Bug fix (non-breaking change that fixes an issue)\n- [x] New feature (non-breaking change that adds functionality)\n- [ ] Breaking change (fix or feature that would cause existing functionality to change)\n- [x] Documentation update\n- [ ] Performance improvement\n- [ ] Code refactoring (no functional changes)\n\n## Changes Made\n\n- Adds HEADROOM_DETACHED_PROFILE and --detached-profile policy support.\n- Adds runtime capability reporting for local-state dependency, feature state, degradation mode, reason, and strict-mode violations.\n- Exposes capabilities through GET /capabilities, /health, /stats, and Prometheus gauges.\n- Tightens stateless startup behavior so file logging is skipped when filesystem writes are disabled.\n- Uses collision-safe temporary probes so capability detection cannot overwrite workspace files.\n- Redacts workspace paths, backend URL credentials, and internal topology from remote capability projections.\n- Registers HEADROOM_DETACHED_PROFILE in the settings store.\n- Documents detached mode in website docs and wiki pages.\n- Merged current headroomlabs-ai/headroom@main; latest head is de0dbad.\n\n## Testing\n\n- [x] Focused detached-mode tests pass locally\n- [x] Linting passes locally\n- [x] GitHub CI is green on the latest head\n- [x] Documentation updated\n- [x] Manual review performed\n\n### Latest Local Validation\n\n ext\nuv run --extra dev python -m pytest tests/test_cli_proxy_env.py tests/test_proxy_capabilities.py -q\n81 passed, 1 skipped\nuv run --extra dev ruff check headroom/proxy/capabilities.py headroom/proxy/helpers.py headroom/proxy/server.py headroom/settings_store.py tests/test_cli_proxy_env.py tests/test_proxy_capabilities.py\ngit diff --check\n\n\n## Current Status\n\nThe branch is current with main; public path/backend disclosures and destructive probe collisions are fixed. GitHub CI is green on latest head de0dbad. No self-approval or merge was performed.\n\n## Real Behavior Proof\n\n- Environment: Local isolated worktree using the repository uv environment, merged with current main.\n- Exact command / steps: Ran targeted Ruff checks plus pytest -q tests/test_proxy_capabilities.py tests/test_cli_proxy_env.py after resolving the documentation conflict.\n- Observed result: Ruff passed; 81 tests passed and 1 skipped. Remote capability and health projections redact the workspace path while loopback operator views retain it.\n- Not tested: Live production deployment probes with production credentials; remote and loopback behavior are covered through TestClient regression tests.\n\n## Runtime Rollout Safety\n\n- Rollout-managed feature(s): detached-mode capability and health projections.\n- Minimum rollout channel: normal release after proxy capability and environment tests are green.\n- Stable/default behavior changed: capability responses gain additive detached-mode metadata with remote redaction preserved.\n- Kill switch / disable path: disable detached operation or human-revert the additive capability changes.\n- Unsafe override required: none.\n- Qualification impact: proxy capability, environment, loopback, and remote-redaction tests must remain green.\n- Rollback path: human revert removes the additive projections; no persistent migration is required.\n\n## Review Readiness\n\n- [x] I have performed a self-review\n- [x] This PR is ready for human review\n- [x] GitHub CI is green on the latest head\n- [x] Blocking review findings are resolved; refreshed CI is green\n\n## Checklist\n\n- [x] My code follows the project's style guidelines\n- [x] I have performed a self-review of my code\n- [x] I have commented hard-to-understand areas where appropriate\n- [x] I have made corresponding documentation changes\n- [x] My changes generate no new local warnings\n- [x] I have added or updated tests for the new behavior\n- [x] Focused unit tests pass locally\n- [x] I have updated CHANGELOG.md if applicable\n\n## Screenshots\n\nN/A\n