[AAASM-4963] 🔧 (ci): Migrate default branch master→main

[Chisanan232]

What changed

Default-branch migration for examples: master → main, per ADR 0016
(AAASM-4963, Epic AAASM-4955). The GitHub rename already moved the default
pointer, branch protection, and installed the master→main redirect. This PR
lands the reference updates the rename does not touch:

• CI branch filters — every workflow's on.{push,pull_request}.branches
  filter repointed master→main (codeql, verify-node/python/go/scenarios,
  example-metadata-check, proof-html). paths:-filtered pull_request
  triggers were already branch-agnostic and needed no change.
• Docs prose — .claude/CLAUDE.md gotcha rewritten (it previously said
  master was default and main was a stale branch — now inverted), the
  sdk-version-bump skill's branch/PR-base pointed at main, the workflow
  README trigger column refreshed, and the cross-repo .github baseline link
  switched to the rename-proof /blob/HEAD/ form.

Audit results:

• Outbound (git grep master): only the 8 workflow branch filters + the
  doc prose above. No hardcoded self-URLs; no CONTRIBUTING/PR-template refs.
• Inbound (org-wide): no release-critical coupling. examples is not in
  agent-assembly/release.yml's bot-PR base: list (that list targets the tap +
  three SDKs only). No raw.githubusercontent.com/.../examples/master (would
  404), no @master action refs, no base: master opening PRs into this repo.
  All inbound refs are github.com/.../examples/{tree,blob,commits}/master web
  links in docs (agent-assembly, python-sdk, node-sdk, go-sdk) and the
  .github org-profile badge — all 301-redirect-covered (cosmetic, non-breaking
  per ADR §6 / Accepted risks), swept separately.

Related ticket

Closes AAASM-4963

How to verify

• gh api repos/ai-agent-assembly/examples -q .default_branch → main.
• Branch protection present on main (1 required approval + code-owner reviews).
• After merge, CI (verify-*, codeql, proof-html) fires on push to main.
  Transitional note: until this PR merges, push-to-main filters do not fire;
  PR-path lanes still run on this PR.

Checklist

• PR title follows [AAASM-XXXX] <GitEmoji> (<scope>): <summary>
• No secrets, API keys, or .env files committed
• Example sub-projects include their own README.md with prerequisites and run instructions
• SDK/runtime version dependencies are documented or pinned

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Chisanan232]

🤖 Claude Code review (evidence only — merge is a Pioneer reviewer's call)

1. CI — ✅ green (34 pass / 0 fail / 1 skip). 2. CodeCov/SonarCloud — ✅ no issues.

3. Scope vs AAASM-4963 — ✅ complete. examples master→main: 8 workflow branch-filters + .claude/CLAUDE.md + sdk-version-bump SKILL + workflows README + a verification-report ref. paths:-only + schedule-only workflows correctly needed no change; cross-repo link → rename-proof /blob/HEAD/.

4. Side effects — ✅ none. Pure branch-reference migration (every removed master replaced by main; zero logic/job/threshold edits). No product/business-logic code touched.

5. Front-end — n/a. No FE source changed (workflows + Markdown + CI config only) → Playwright not applicable.

Verdict: ready for approval + merge. Not self-merged.