DOC: Add prose-budget for committed text and warning-level commit-msg hook

[hjmjohnson]

Codify hard caps for committed text and add a warning-level commit-msg hook so reviewers stop being asked to redo the same "trim the prose" round-trip on every PR.

What this PR changes

Two new docs, one new hook, three doc edits, one config wire-in.

File	Status	Purpose
Documentation/AI/prose-budget.md	new	Single authoritative contract: caps, forbidden content, three-layer rule (subject = WHAT, body = WHY, diff = HOW), self-audit checklist, good/bad examples.
Documentation/AI/code-comments.md	new	In-source comment policy: default zero comments, max one short line, WHY-only, no references to deleted code, no documentation-comment drive-bys.
Documentation/AI/community-feedback.md	new	Anonymized ledger of recurring reviewer-rejection patterns, each linked to the document and section that addresses the rule.
Documentation/AI/git-commits.md	edited	Adds Body budget section (≤ 12 lines, ≤ 72 chars/line, trailers excluded) with forbidden-content list and good/bad commit examples.
Documentation/AI/pull-requests.md	edited	Adds Visible-summary budget section (≤ 3 sentences, ≤ 250 chars above the first <details>) with forbidden-content list.
AGENTS.md	edited	Adds two MUST-READ entries for any committed-text task and a Critical Pitfall #7 promoting verbose prose to ship-blocker status.
Utilities/Hooks/kw-prose-budget.py	new	Commit-msg hook (warning-level by default, blocking under ITK_PROSE_BUDGET_HARD=1) flagging body-length / line-width / forbidden-pattern violations.
.pre-commit-config.yaml	edited	Wires the new hook adjacent to kw-commit-msg.

Caps at a glance

Artifact	Cap
In-source comment	1 short line, ≤ 100 chars
Commit subject	≤ 78 chars (already enforced by kw-commit-msg.py)
Commit body	≤ 12 lines, ≤ 72 chars/line; trailers excluded
PR visible summary	≤ 3 sentences, ≤ 250 chars
PR <details> blocks	unbounded (collapsed)
PR HTML comments	unbounded (invisible)

Forbidden in committed text

• AI tool / model identifiers, session timestamps
• CDash / Azure DevOps / GitHub Actions URLs (transient — links rot)
• Backstory / transcript narratives
• References to deleted or prior behavior
• First-person narrative ("we", "I")
• Marketing language ("robust", "comprehensive", "elegant", …)
• Restatements of the commit subject in the first body line
• Explanations of code that the diff already shows

Hook behavior

The new kw-prose-budget.py runs at commit-msg stage alongside kw-commit-msg.py. By default it prints warnings and exits 0 (commit allowed); set ITK_PROSE_BUDGET_HARD=1 to escalate to blocking. The patterns are conservative — reviewers remain the authoritative gate; the hook reduces round-trips by surfacing the most-cited rejection patterns at commit time.

Smoke-tested locally on a synthetic verbose body — flags transient URLs, AI tool names, marketing language, line-width and line-count caps; allows the commit through with a 1 warning(s); commit allowed. summary line.