feat: JSON multi-profile runtime config for Gitea MCP (roadmap #10)

Let one MCP server select among named Gitea runtime profiles from a JSON file
instead of editing code or juggling many .env files:

    GITEA_MCP_CONFIG=/path/to/gitea-mcp.json
    GITEA_MCP_PROFILE=dev

- New gitea_config.py: load/validate the JSON, select the named profile, and
  resolve its token by env-var reference. Profiles supply base_url,
  profile_name, token_env, owner/repo, allowed/forbidden operations, and audit
  label.
- gitea_auth.get_profile() now overlays env over the selected JSON profile:
  explicit env vars win, the JSON profile fills only what env leaves unset.
- gitea_auth.get_auth_header() gains a JSON token_env fallback after explicit
  env tokens (env still wins).

Security / safety:
- Tokens are referenced by env-var NAME (token_env); an inline "token" is
  rejected and never echoed. The value is never stored in or returned as
  profile metadata.
- Fail-safe errors: missing file / invalid JSON / unknown or unset selected
  profile raise a clear ConfigError that never prints file contents or tokens
  (JSONDecodeError context is suppressed so the raw file text can't surface).
- No network calls during config parsing.
- Real config files are gitignored (gitea-mcp*.json), example kept.

Backwards compatible: with GITEA_MCP_CONFIG unset, behaviour is exactly the
prior env-only behaviour (all existing get_profile/get_auth_header tests pass
unchanged).

Docs: README JSON-profiles section + env table rows, .env.example placeholders,
gitea-mcp.example.json.
Tests: tests/test_config.py (22 cases) — env-only, selection, multiple
profiles, env-override precedence, missing file, invalid JSON, missing/unset
profile, inline-token rejection + redaction, and no-network-during-parse.

Refs #10. Note: issue #19 (env-based profiles) was already implemented and
closed; this JSON-file capability is adjacent new scope tracked under the
roadmap rather than reopening #19.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -169,6 +169,8 @@ Recognized environment fields (see [`.env.example`](.env.example) for placeholde
 | `GITEA_TOKEN_SOURCE` | Optional *name* of the token source (e.g. an env var name). A name only — never the token value. |
 | `GITEA_BASE_URL` | Optional informational base URL. |
 | `GITEA_AUDIT_LOG` | Optional path to an audit log file. When set, mutating actions append one redacted JSON record each (profile + authenticated user + outcome). Unset ⇒ auditing off (no records, no extra API calls). |
+| `GITEA_MCP_CONFIG` | Optional path to a JSON file defining multiple named runtime profiles. Unset ⇒ pure env behaviour. |
+| `GITEA_MCP_PROFILE` | Name of the profile (from `GITEA_MCP_CONFIG`) to activate for this runtime. |
 
 Notes:
 
@@ -187,6 +189,41 @@ Notes:
   branch and head SHA where applicable — when `GITEA_AUDIT_LOG` is set. Auditing
   is off by default and never adds API calls or breaks the action when off.
   See [`gitea_audit.py`](gitea_audit.py).
+
+**JSON runtime profiles (roadmap #10).** Instead of juggling many `.env` files,
+one server can pick a named profile from a JSON file (see
+[`gitea-mcp.example.json`](gitea-mcp.example.json), loaded by
+[`gitea_config.py`](gitea_config.py)):
+
+```bash
+export GITEA_MCP_CONFIG=/path/to/gitea-mcp.json
+export GITEA_MCP_PROFILE=dev
+```
+
+```json
+{
+  "profiles": {
+    "dev": {
+      "base_url": "https://gitea.dev.example",
+      "profile_name": "gitea-author",
+      "token_env": "GITEA_TOKEN_DEV",
+      "allowed_operations": ["read", "pr.create"],
+      "audit_label": "dev-author"
+    }
+  }
+}
+```
+
+- **Token by reference only:** a profile names the env var holding its token
+  (`token_env`); an inline `token` is rejected. The value is read from that env
+  var and never stored in or returned as profile metadata.
+- **Precedence:** explicit env vars (`GITEA_PROFILE_NAME`, `GITEA_BASE_URL`,
+  `GITEA_ALLOWED_OPERATIONS`, `GITEA_TOKEN`, …) **override** the JSON profile;
+  the JSON profile only fills what the environment leaves unset.
+- **Backwards compatible / fail-safe:** with `GITEA_MCP_CONFIG` unset, behaviour
+  is exactly env-only. A missing file, invalid JSON, or unknown/unset selected
+  profile raises a clear startup error that never prints file contents or
+  tokens. Parsing makes no network calls.
 </details>
 
 <details>