chore: pin ai-workflows self-references at @main

[JacobPEvans-personal]

What

Aligns this repo with the org self-reference convention: every
dryvist/ai-workflows/… reference rides @main, never a SemVer tag —
matching the now-fixed terraform-proxmox wrapper
(terraform-proxmox#502)
and documented in docs#89 /
dryvist/.github#52.

Changes

• examples/*-caller.yml — @v0 → @main (the canonical copy-paste
  templates).
• dogfood-*.yml — @v0.16.0 → @main (live self-references that dogfood
  this repo's own workflows).
• suite-*.yml — usage comments + the transitivity note → @main.
• README / AGENTS / GETTING_STARTED / PATTERNS — caller examples → @main.
• GETTING_STARTED — new authoritative Versioning note explaining the
  @main convention and linking to the docs CI/CD policy scanner posture.

release-please and the moving major/minor tags are untouched — SemVer tags
keep existing for the changelog and per-run resolved-SHA audit trail, but
consumers ride @main so upstream fixes land with no per-repo Renovate PR.

Verification

• bun test → 158 pass, 0 fail (no script logic changed).
• grep confirms zero remaining ai-workflows/…@vX references in the repo.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates all documentation and example workflows to reference the @main branch instead of SemVer tags or SHAs, and adds a section explaining this versioning policy. The review feedback correctly points out that using the term 'pin' when referring to a mutable branch like @main is technically a contradiction and suggests using 'reference' or 'use' instead to avoid confusion.