Rename sync.sh to sync.py to reflect Python implementation

[sebastientaggart]

Rename sync.sh to sync.py so editors, linters, and GitHub's language detection correctly identify the sync engine as Python. Updates all references across source skills, documentation, CI workflows, config files, templates, and the Makefile. Adds "generated by CodeCannon/sync.sh" as a legacy marker so existing generated files in consumer projects are still detected and properly regenerated. Also adds a Python 3.8+ note to the quick-start instructions.

Issue #122

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

No issues found. Code looks correct and follows project conventions.

The rename from sync.sh to sync.py is applied consistently across all 53 changed files. The only substantive code change is in sync.py itself:

• LEGACY_MARKER (single string) becomes LEGACY_MARKERS (list), correctly adding "generated by CodeCannon/sync.sh" as a legacy marker so existing generated files in consumer projects are detected and regenerated on the next sync run.
• needs_migration logic correctly triggers re-write when a file has a legacy marker string (not just legacy position), ensuring the old sync.sh marker lines get replaced.
• All documentation, CI workflows, Makefile targets, templates, and generated skill files reference sync.py.
• The Python 3.8+ note in README and docs/index.md is accurate (the script uses stdlib only).

No remaining stale sync.sh references outside the intentional LEGACY_MARKERS list in sync.py.