feat: load profiles.json v2 contexts shape with enabled enforcement and LLM-safe output (#120)

Support the canonical contexts-shape version 2 config (contexts / profiles /
projects / rules) alongside the existing environments shape and v1:

- Require a boolean 'enabled' on every context, profile, service, and
  project. Disabled entries are surfaced in audits but fail closed at
  selection/resolution — never a silent fallback to another profile,
  service, or credential source.
- Resolve the active identity from GITEA_MCP_PROFILE via the existing
  select_profile path; profile base_url falls back to the context's enabled
  gitea block.
- Add resolve_service() and project_for_path() for context service and
  project-to-context resolution (internal use; fail closed on disabled).
- get_auth_header now propagates ConfigError when GITEA_MCP_CONFIG is set
  instead of silently degrading to Basic auth.
- Hide endpoint URLs and keychain ids from normal LLM-facing output:
  gitea_whoami / gitea_get_profile report logical names and auth status
  only; new gitea_audit_config tool reports enabled/disabled state and safe
  one-line service summaries. The GITEA_MCP_REVEAL_ENDPOINTS opt-in (and
  'python3 gitea_config.py audit --reveal-endpoints' locally) restores
  endpoints and auth source names for admin diagnostics; token values are
  never printed on any path.
- Ship gitea-mcp.v2-contexts.example.json (synthetic values) and validate
  it in tests.

Implements #120

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gitea-mcp.v2-contexts.example.json

@@ -0,0 +1,80 @@
+{
+  "version": 2,
+  "contexts": {
+    "example-context": {
+      "enabled": true,
+      "label": "Example environment",
+      "description": "One deployment environment: its Gitea plus non-Gitea services.",
+      "default_owner": "Example-Org",
+      "gitea": {
+        "enabled": true,
+        "kind": "gitea",
+        "base_url": "https://gitea.example.invalid"
+      },
+      "services": {
+        "jenkins": {
+          "enabled": true,
+          "kind": "jenkins",
+          "label": "Example Jenkins",
+          "base_url": "https://jenkins.example.invalid",
+          "auth": { "type": "keychain", "id": "example-jenkins-token" },
+          "capabilities": ["read"]
+        },
+        "glitchtip": {
+          "enabled": false,
+          "kind": "glitchtip",
+          "label": "Example GlitchTip (disabled: defined but unavailable)",
+          "base_url": "",
+          "auth": { "type": "keychain", "id": "example-glitchtip-token" },
+          "capabilities": ["read"],
+          "allow_raw_events": false
+        }
+      }
+    }
+  },
+  "profiles": {
+    "example-author": {
+      "enabled": true,
+      "context": "example-context",
+      "role": "author",
+      "username": "author-user",
+      "execution_profile": "example-author",
+      "audit_label": "example-author",
+      "auth": { "type": "keychain", "id": "example-gitea-author-token" },
+      "allowed_operations": ["read", "branch", "commit", "push", "open_pr", "comment"],
+      "forbidden_operations": ["approve", "request_changes", "merge"]
+    },
+    "example-reviewer": {
+      "enabled": true,
+      "context": "example-context",
+      "role": "reviewer",
+      "username": "reviewer-user",
+      "execution_profile": "example-reviewer",
+      "audit_label": "example-reviewer",
+      "auth": { "type": "keychain", "id": "example-gitea-reviewer-token" },
+      "allowed_operations": ["read", "review", "comment", "approve", "request_changes", "merge"],
+      "forbidden_operations": ["branch", "commit", "push", "open_pr"]
+    }
+  },
+  "projects": {
+    "/absolute/path/to/local/repo": {
+      "enabled": true,
+      "context": "example-context",
+      "default_owner": "Example-Org",
+      "default_repo": "Example-Repo",
+      "default_author_profile": "example-author",
+      "default_reviewer_profile": "example-reviewer"
+    }
+  },
+  "rules": {
+    "disabled_behavior": "Defined but unavailable for action. MCP tools may report disabled entries during audits, but must not use them automatically.",
+    "no_silent_fallback": true,
+    "tokens_in_json": false,
+    "token_storage": "keychain",
+    "identity_must_match_task": true,
+    "same_username_cannot_review_own_pr": true,
+    "hide_service_urls_from_llm": true,
+    "hide_keychain_ids_from_llm": true,
+    "mcp_resolves_endpoints": true
+  }
+}