Remove the --deliberate run mode per the #442 kill decision

[solomonneas]

#442 closed with an operator decision to kill the opt-in grounded-deliberation flag. The mode was merged in #452 and its planner bug was fixed in #470, so it is being removed in working order and stays recoverable from history.

Scope:

• Remove the --deliberate flag from brigade run and its plumbing.
• Remove the deliberation planner/scope machinery in src/brigade/deliberation.py and the brigade.deliberation.v1 artifact emission, keeping any helpers that other code paths use.
• Remove or retarget the deliberation tests, including the probe-flag regression test from fix(deliberation): stop passing --markdown/--limit to graph probe commands #470 if its subject goes away with the mode.
• Update docs that mention the flag.

Keep docs/deliberation-experiment.md and runs/experiment/ on the experiment branch as the record; they are referenced from #442.

Refs #442.