fix(cli,mcp): UTF-8 stderr, a stdout guard, and ~ in OSW_CRED_FILEPATH

[LukasGold]

Follow-ups to #133, now that it has merged.

#194 - a leading ~ in OSW_CRED_FILEPATH was not expanded

• Settings._validate_cred_filepath expands ~, as _validate_state_dir already does.
• _resolve_cred_file expands it too. The issue's suggested fix alone is not enough: load() calls Path(cred_filepath).is_file() on _resolve_cred_file's return value (src/osw/service/config.py) long before it builds Settings, so the field validator runs too late. ~/accounts.pwd.yaml was reported as missing while the file was there.
• The expansion happens after _cred_file_var is determined, because that lookup matches the raw value against os.getenv.
• A relative path stays accepted, unlike state_dir. The CLI resolves accounts.pwd.yaml against the working directory on purpose.
• expanduser() raises RuntimeError when no home directory can be found. Both call sites catch it and name the variable, so the message is actionable. load()'s Raises docstring lists the new case.
• 5 tests in tests/test_service_config.py; 4 of them fail on the unfixed source.

#193 - the osw-mcp startup report was written with the locale encoding

• New src/osw/service/streams.py with force_utf8(*streams). The CLI had this logic inline in _force_utf8_output; it now lives in the shared core, because osw.mcp may never import osw.cli (tests/test_no_paths_on_mcp_surface.py).
• main() in src/osw/mcp/server.py calls it on stderr only. An MCP client starts the server with stderr on a pipe, so Python picks the locale encoding, cp1252 on a German Windows system. The report carries the credential file path and the env file path.
• stdout is deliberately left alone. The SDK's stdio_server re-wraps the binary buffer as UTF-8 itself and claims file descriptor 1 while doing it.
• reconfigure() is passed errors= explicitly. It silently resets the handler to strict otherwise, and a strict stderr would raise while reporting a failure.
• Reconfiguring in place also covers osw's own log handler, which holds the same stream object.

#192 - no test held stdout free of log records during a tool call

• tests/test_mcp_server.py now calls the real status tool through mcp.call_tool("status", {}) and asserts stdout is empty while the record appears on stderr.
• status is used because it logs a warning from inside ctx.guard() when the connection check fails (src/osw/service/ops/status.py).
• Two mechanisms keep stdout clean and only one is osw's own code: enable_logging defaults its handler to sys.stderr, and the SDK claims file descriptor 1. A single edit to that default would undo the first, which is what the test holds. It fails when src/osw/__init__.py is changed to sys.stdout.
• The test resets the root handlers and re-runs enable_logging in a context manager, not a fixture, because pytest attaches its log-capture handler after fixtures run. Mirrors plain_logging in tests/test_logging_setup.py.

Verification

• Full unit suite: 668 passed.
• make check: lock file, pre-commit, ty, deptry all pass.
• Each fix was proven by reverting the source change and watching the new test fail.
• Run on Windows only.

[github-actions]

Release preview

Merging this PR would release v2.6.1 (current: v2.6.0).

Changelog preview (truncated)

## v2.6.1 (2026-09-21)

### Bug Fixes

- **config**: Expand a leading ~ in OSW_CRED_FILEPATH
  ([`eb35ba7`](https://github.com/OpenSemanticLab/osw-python/commit/eb35ba7bdf5be6e9a10f108fcc5f7c10133d6515))

- **mcp**: Encode the osw-mcp startup report as UTF-8
  ([`979ad81`](https://github.com/OpenSemanticLab/osw-python/commit/979ad81ea20e2c0b85444c8f49d3815c5029ff3a))

_{Preview via python-semantic-release and conventional commits.}