docs: adopt gbrain's release-summary CHANGELOG format + apply to v1.3

Ported the "release-summary format" rules from ~/git/gbrain/CLAUDE.md
(lines 291-354) into gstack's CLAUDE.md under the existing
"CHANGELOG + VERSION style" section. Every future `## [X.Y.Z]` entry
now needs a verdict-style release summary at the top:
1. Two-line bold headline (10-14 words)
2. Lead paragraph (3-5 sentences)
3. "Numbers that matter" with BEFORE / AFTER / Δ table
4. "What this means for [audience]" closer
5. `### Itemized changes` header
6. Existing itemized subsections below

Rewrote v1.3.0.0 entry to match. Preserved every existing bullet in
Added / Changed / Fixed / For contributors (no content clobbered per
the CLAUDE.md CHANGELOG rule).

Numbers in the v1.3 release summary are verifiable — every row of the
BEFORE / AFTER table has a reproducible command listed in the setup
paragraph (git log, bun test, grep for wiring status). No made-up
metrics.

Also added the gbrain "always credit community contributions" rule to
the itemized-changes section. `Contributed by @username` for every
community PR that lands in a CHANGELOG entry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -397,6 +397,60 @@ CHANGELOG.md is **for users**, not contributors. Write it like product release n
 - No jargon: say "every question now tells you which project and branch you're in" not
   "AskUserQuestion format standardized across skill templates via preamble resolver."
 
+### Release-summary format (every `## [X.Y.Z]` entry)
+
+Every version entry in `CHANGELOG.md` MUST start with a release-summary section in
+the GStack/Garry voice, one viewport's worth of prose + tables that lands like a
+verdict, not marketing. The itemized changelog (subsections, bullets, files) goes
+BELOW that summary, separated by a `### Itemized changes` header.
+
+The release-summary section gets read by humans, by the auto-update agent, and by
+anyone deciding whether to upgrade. The itemized list is for agents that need to
+know exactly what changed.
+
+Structure for the top of every `## [X.Y.Z]` entry:
+
+1. **Two-line bold headline** (10-14 words total). Should land like a verdict, not
+   marketing. Sound like someone who shipped today and cares whether it works.
+2. **Lead paragraph** (3-5 sentences). What shipped, what changed for the user.
+   Specific, concrete, no AI vocabulary, no em dashes, no hype.
+3. **A "The X numbers that matter" section** with:
+   - One short setup paragraph naming the source of the numbers (real production
+     deployment OR a reproducible benchmark, name the file/command to run).
+   - A table of 3-6 key metrics with BEFORE / AFTER / Δ columns.
+   - A second optional table for per-category breakdown if relevant.
+   - 1-2 sentences interpreting the most striking number in concrete user terms.
+4. **A "What this means for [audience]" closing paragraph** (2-4 sentences) tying
+   the metrics to a real workflow shift. End with what to do.
+
+Voice rules for the release summary:
+- No em dashes (use commas, periods, "...").
+- No AI vocabulary (delve, robust, comprehensive, nuanced, fundamental, etc.) or
+  banned phrases ("here's the kicker", "the bottom line", etc.).
+- Real numbers, real file names, real commands. Not "fast" but "~30s on 30K pages."
+- Short paragraphs, mix one-sentence punches with 2-3 sentence runs.
+- Connect to user outcomes: "the agent does ~3x less reading" beats "improved precision."
+- Be direct about quality. "Well-designed" or "this is a mess." No dancing.
+
+Source material:
+- CHANGELOG previous entry for prior context.
+- Benchmark files or `/retro` output for headline numbers.
+- Recent commits (`git log <prev-version>..HEAD --oneline`) for what shipped.
+- Don't make up numbers. If a metric isn't in a benchmark or production data,
+  don't include it. Say "no measurement yet" if asked.
+
+Target length: ~250-350 words for the summary. Should render as one viewport.
+
+### Itemized changes (below the release summary)
+
+Write `### Itemized changes` and continue with the detailed subsections (Added,
+Changed, Fixed, For contributors). Same rules as the user-facing voice guidance
+above, plus:
+
+- **Always credit community contributions.** When an entry includes work from a
+  community PR, name the contributor with `Contributed by @username`. Contributors
+  did real work. Thank them publicly every time, no exceptions.
+
 ## AI effort compression
 
 When estimating or discussing effort, always show both human-team and CC+gstack time: