fix(ci): openapi generator merge-preserves hand-authored paths

[bokelley]

Why

The TypeScript Build CI step has been red on main since #4771 merged. That PR added 685 lines of brand-registry endpoint docs directly to `static/openapi/registry.yaml` because the routes are docs-only — they exist in Express but were never given Zod schemas. The freshness lint regenerates the yaml from Zod and `git diff --exit-code`s, so the hand-authored content gets clobbered on every regen and fails.

Verified currently failing on main:

```
gh api repos/adcontextprotocol/adcp/commits/main/check-runs --jq '.check_runs[] | select(.name | contains("TypeScript Build")) | .conclusion'

→ failure

```

Every open PR carries the inherited red regardless of whether it touched openapi at all.

The fix

Generator now merge-preserves hand-authored content:

1. Read existing `registry.yaml` from disk before emit.
2. For each path / component schema / tag in the existing yaml that's NOT in the generator's tracked output, copy it forward.
3. Generator wins on conflicts so Zod-backed paths remain authoritative.

This means docs teams can keep landing pure-yaml PRs (the brand-registry case, and any future docs-only surfaces) without first wiring Zod schemas, AND the freshness lint stops blocking unrelated PRs.

Also moved `Brand Logos` and `Brand Wiki` tag descriptions into `TAG_DESCRIPTIONS` so they emit between `Brand Resolution` and `Property Resolution` (their documented position) rather than appended at the tag list's tail.

Why not write Zod schemas for the brand-registry surface?

That's the structurally correct fix and worth doing eventually. But it's 9 operations with multipart upload + 4 error variants each + moderator queue + ownership endpoint — real engineering, separate scope. Filing this PR unblocks repo-wide CI today.

Test plan

• WORKOS_API_KEY=test_dummy WORKOS_CLIENT_ID=client_test npm run build:openapi runs cleanly
• git diff --exit-code static/openapi/registry.yaml clean after the regen
• Brand-registry paths still present in regenerated yaml (preserved by the merge)
• Tag order matches TAG_DESCRIPTIONS (Brand Logos / Brand Wiki between Brand Resolution and Property Resolution)
• npm run test:schemas / npm run test:json-schema clean (this PR only touches the generator and registry.yaml)

Followup

A separate issue should track wiring real Zod schemas for the brand-registry surface so they're code-authoritative and inspectable. Not blocking this fix.

🤖 Generated with Claude Code

[bokelley]

Closing — superseded by #4793 which took the policy answer (hand-edited yaml entries are not supported; the path is Zod schemas, not workarounds in the generator).

This PR would have made the freshness lint tolerate hand-edited paths by merge-preserving them on regen. That's defensively useful, but it also re-enables the exact pattern #4793 deliberately closed: it lets future contributors bypass writing Zod schemas for new routes, which is the right long-term answer.

The TOCTOU fix on existsSync/readFileSync in this PR is moot too, since the merge-preserve code that introduced the race is being dropped.

If a future need for yaml-first docs surfaces, it should come with that policy conversation rather than silently being unblocked by this PR's machinery.