feat: Workflow Template Registry — remote discovery, publish validation

[Copilot]

Adds a remote registry backed by a GitHub repo for discovering and sharing community workflows, extending the existing local template infrastructure.

New CLI commands

conductor templates --remote                              # list community templates
conductor init my-project -t registry:research-pipeline   # scaffold from registry
conductor publish my-workflow.yaml                        # validate before sharing

Changes

• src/conductor/cli/registry.py — New module: registry client (index fetching, template download, render), publish validation with security checks (suspicious shell patterns, hardcoded secrets, schema validation)
• src/conductor/cli/app.py — templates gets --remote flag, init gains registry: prefix routing, new publish command
• tests/test_cli/test_registry.py — 37 tests covering registry fetch/render, publish validation, CLI integration; all 325 existing CLI tests unaffected

Design notes

• Registry is a simple GitHub repo (microsoft/conductor-workflows) with a JSON index and YAML templates — no new infrastructure
• publish validates locally and directs users to open a PR against the registry repo; no write API
• Security gate catches curl|sh, rm -rf /, hardcoded API keys, eval()/exec() patterns
• Network failures are non-fatal for templates --remote (shows error, exits 0)