docs: index shared automation source links

[rianjs]

Summary

• add a repo-level automation index for shared actions and reusable workflows
• document the canonical GitHub source plus local convenience copy reference pattern
• point CI/release/distribution policy back to cli-common docs without duplicating mechanics

Verification

• git diff --cached --check
• rg -n -F "Source of truth:" README.md
• rg -n -F "Local convenience copy, if present:" README.md
• rg -n -F checks for the shared action and reusable workflow examples

Closes #19

[rianjs]

No findings.

The PR matches the intended scope for ticket 2: it adds a root automation index, uses the cli-common#44 Source of truth: / Local convenience copy, if present: pattern, covers both composite actions and reusable workflows, and points CI/release/distribution policy back to cli-common without duplicating mechanics. The diff is docs-only and does not affect profile content or automation behavior.