Route structured logs to a file instead of the console

[TheGreatAxios]

Summary

• A raw structured log line from a vendored logger (interchange.inference.default-director) could reach stdout mid-frame and corrupt the TUI's alternate screen, because @intx/log installs a console sink as an import side effect and nothing in Corbits ever replaced it.
• Add installFileLogSink (src/logging/sink.ts) and call it as the first statement of mainWithRunners, before loadConfig or anything else can log, so every logger — including ones inside vendor/ — writes to ~/.corbits/logs/corbits.log instead of the terminal.
• The file sink's floor is debug, not warning: a file has no screen to corrupt, and a dozen existing logger.debug calls in src/tui/runner.ts and src/exec/runner.ts exist specifically to diagnose teardown races. A higher floor would have silently made those unreachable.
• No change to error presentation: the readable, sentence-form error was already rendered elsewhere in the transcript; this only removes the duplicate raw log line that used to leak onto the screen.

Verification

• bun run typecheck
• bun run build
• bun run test (3968 pass)
• New test drives the real OpenTUI renderer (src/tui-opentui/log-sink.test.ts): mounts the shell, fires the vendored logger's exact call shape, and asserts the captured frame is unchanged, process.stdout.write/process.stderr.write are never called, and the record lands in the log file.
• The riskiest property — that no module-scope getLogger call in the reachable import graph actually logs before installFileLogSink() runs — was verified by tracing the import graph rather than covered by an automated test; a mock-based ordering test was attempted and dropped after it deadlocked bun's module-mock cache. A subprocess test spawning the real entry point and asserting nothing reaches stdout would close this gap and is a reasonable follow-up.

Known follow-ups (not fixed here)

• src/logging/sink.ts appends to the log file forever with no rotation or size cap.
• The hand-rolled LogRecord type in src/logging/sink.ts structurally mirrors @logtape/logtape's real type (two packages away, not a declared dependency) rather than importing it. It typechecks today — the real type is strictly wider — but nothing catches drift if a future LogTape version renames or narrows a field this file reads.

Closes CL-5593

[linear-code]

CL-5593