docs(cross-project-broadcast): codify capability-and-rule cadence guideline

Adds one bullet to When NOT to Use and a new Cadence Guideline
section that names the per-commit-broadcast anti-pattern explicitly.
The new section lays out the five reasons broadcasts stay
capability-and-rule-level rather than commit-level: cost per
broadcast across the fleet, signal-to-noise on project-internal
commits, the rsync-already-does-the-work observation, aggregation
winning over per-commit pings, and the train-projects-to-ignore-inbox
risk.

The end-of-session bundling guidance lands too. If a session ships
several broadcastable changes, bundle them into one broadcast at
session end instead of firing one per commit.

Source: session-end conversation 2026-05-29 surveying today's
cross-project changes. Today's session shipped 13 cross-project
items (4 new workflows, 6 workflow updates, 2 new scripts, 1 new
bin tool). A per-commit broadcast cadence would have fired 13 inbox
files across 23 targets, or 299 total inbox files. One consolidated
broadcast (or no broadcast at all, since Craig already coordinated
manually) covered the same ground.

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