skills/design-issue: automatically match existing feature design documents on Epics for Major Revisions

[JoshuaVSherman]

Child of web-jam-tools#737 #737 ("design-issue skill enhancements and fixes").

What this builds

Updates skills/design-issue/SKILL.md and src/design-issue/ CLI resolution helpers to automatically detect pre-existing canonical design documents in ~/Dropbox/web-jam-llms/<Theme>/ when invoked on an Epic or feature, proposing a Major Revision to the existing document rather than spawning a new standalone file.

1. Enhances design document candidate detection in src/design-issue/gate1.ts and src/design-issue/cli.ts:
   • Scans ~/Dropbox/web-jam-llms/<Theme>/ for existing topic design documents (<topic>-design-*.md or <topic>-skill-design-*.md).
   • If a matching document exists for the feature/skill, resolves to that existing document and suggests a Major Revision.
2. Updates skills/design-issue/SKILL.md:
   • Codifies the Major Revision workflow: records a new entry in ## Revision History, updates architecture, ERD, and decisions in-place, preserving the document as the single source of truth for the feature.
3. Adds automated unit test coverage in test/design_issue_gate1.test.ts.

Design reference

~/Dropbox/web-jam-llms/Token_Savings/design-issue-enhancements-design-2026-08-23.md

STOP and say so if this path cannot be read. Do not reconstruct the requirements from this issue body, from comments, or from guesswork.

Acceptance criteria

1. Running /design-issue or design:candidates on an Epic whose feature already has a canonical design document automatically resolves to the existing document and prompts for a Major Revision.
2. The skill refuses to create redundant parallel design documents for the same feature.
3. skills/design-issue/SKILL.md documents the automatic feature matching and Major Revision protocol.
4. Unit tests in test/design_issue_gate1.test.ts verify existing document discovery and path resolution.
5. deno task test, deno task fmt:check, deno task lint, and deno task sast pass 100% green.