Prevent persisted run history for unknown commands

[codeforester]

Fixes #1690

What changed

• Validate the top-level basectl command before initializing persistent run bundles.
• Keep the existing usage error and exit code for unknown commands without writing run.json, primary.log, or history.
• Preserve run bundles for recognized commands that fail during leaf validation, environment checks, or execution.
• Document the accepted-invocation boundary in the observability guide.

Root cause

The dispatcher initialized the run bundle before its final command case rejected unknown commands, so invalid invocations were recorded as failed runs.

Validation

• Focused runtime-dispatch.bats: 27 passed.
• Full Python suite: 1,019 passed, 1 skipped.
• Real CLI smoke check confirms basectl invalid exits 2 and creates no cache state.
• ShellCheck and git diff --check pass.