run --local: attach a file log subscriber so application logs are captured

[ako]

Summary

Round 2 of findings #25. PR #38 tee'd the runtime JVM's stdout/stderr to .mxcli/runtime.log, but a re-test on the Sudoku app showed the file stays nearly empty: microflow LOG output and server-side exception stack traces never appear, so a page-action error still can't be diagnosed (the browser only shows the generic Mendix dialog).

Root cause: a standalone runtime launched via runtimelauncher.jar attaches no log subscriber by default — unlike a Studio Pro / m2ee run, which calls create_log_subscriber after start. Mendix application logs flow to log subscribers, not stdout, so with none attached they go nowhere.

Fix

After a successful start (and on every restart's Start, since each fresh JVM has no subscriber), attach a file log subscriber via the create_log_subscriber admin action, mirroring m2ee-tools' post-start _configure_logging:

{ type: "file", name: "mxcli-run-local", autosubscribe: "INFO",
  filename: <abs runtime.log>, max_size: 1 GiB, max_rotate: 0 }

• max_rotate: 0 is load-bearing — the JVM stdout tee (PR run --local: tee the Mendix runtime log to a file for debugging #38) holds an open fd on the same file, and a rotate-rename would detach it. Both writers append.
• The path is passed absolute (the runtime's cwd is <install>/runtime, not mxcli's).
• Best-effort: a logging failure warns to stdout rather than failing an otherwise-healthy start. Disabling the file with --runtime-log - disables the subscriber too.

Tests

• TestStart_AttachesLogSubscriber — asserts the params (type/filename/autosubscribe/name/max_rotate:0) after a successful start.
• TestStart_NoLogSubscriberWhenUnset — no subscriber when LogSubscriberFile is empty.
• TestStart_LogSubscriberFailureNonFatal — a failing create_log_subscriber does not fail Start; it warns instead.

All docker unit tests pass; the only failure is the pre-existing environmental TestServeIntegration (test app 11.6.3 vs MxBuild cache 11.12.1), unrelated to this change.

Docs

• run help text, run-local skill, and docs-site "Debugging a server-side error" now explain that two sources feed runtime.log (JVM tee + file log subscriber) and why.
• fix-issue.md symptom table extended with the round-2 entry.

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