feat(registry): tag-based versioning with # ref syntax and CDN-bypass

[jrob5756]

Summary

Reworks the workflow registry to behave more like a plugin system — versions are auto-discovered from git tags instead of being explicitly listed, refs can pin to any tag/branch/SHA, and stale CDN content is no longer a problem.

Closes the two pain points raised in discussion:

1. `conductor registry update` doesn't reflect changes immediately — root cause was Fastly's CDN cache on `raw.githubusercontent.com`. Index fetches now resolve the ref to a commit SHA via the GitHub API and fetch from `…//index.yaml`. Unique URL per commit → CDN can't serve stale data. No `--force` flag needed.

2. Versioning was awkward — required maintainers to enumerate every release in `index.yaml`. Now versions are auto-discovered from the registry repo's git tags, semver-sorted.

New ref grammar

```
[@][#]
```

Example	Resolves to
`qa-bot`	default registry, latest tag (or default branch HEAD if no tags)
`qa-bot#v1.2.3`	default registry, ref `v1.2.3`
`qa-bot#main`	default registry, branch `main`
`qa-bot@team`	registry `team`, latest
`qa-bot@team#v1.2.3`	registry `team`, tag `v1.2.3`
`qa-bot@#abc123`	default registry, commit `abc123`

Hard errors: empty ref after `#`, multiple `#` or `@`, empty workflow name, path registry with a ref.

Behavior changes

• `versions: [...]` field dropped from `index.yaml` schema. Existing indexes with this field still parse (Pydantic ignores it) — no migration required.
• `latest` = newest semver-sorted tag (with leading `v` stripped for parsing). Falls back to default branch HEAD if no tags exist.
• Cache key is commit SHA (`//<sha[:12]>/`). Mutable branch refs re-resolve fresh on every fetch; cache hit only when SHAs match.
• Atomic cache writes: fetch into `tempfile.mkdtemp`, then `os.replace` into the final dir. No partial-state visibility for concurrent runs.
• Index and workflow always come from the same commit (`load_index(entry, ref=sha)`) — fixes a subtle bug where `latest` could resolve to a tag that doesn't contain a workflow listed in the default-branch index.
• `registry list` / `registry show` drop the per-workflow "Versions" column and add a "Latest tags:" footer (up to 5 newest, with `, ...` if more) for github registries.
• `registry update` help text updated to explain the SHA-based CDN bypass.

Implementation

Module	Change
`registry/github.py`	Added `get_default_branch`, `resolve_ref_to_sha`; `list_tags` now paginates (cap 1000)
`registry/version_resolver.py`	New module: `resolve_ref` (latest tag → default branch fallback) and `materialize_to_sha`
`registry/index.py`	`load_index(entry, ref=None)` accepts an optional ref; github path always resolves to SHA before fetching
`registry/resolver.py`	New `#`-based parser; `ResolvedRef.version` → `ResolvedRef.ref`
`registry/cache.py`	Ref-aware, SHA-keyed, atomic fetch_workflow
`cli/registry.py`	Drop Versions column, add Latest tags footer
`cli/app.py`	Update 4 callers from `.version` → `.ref`

Tests

• 153 registry tests pass (including 14 new tests in `test_version_resolver.py`).
• Full suite: 2324 pass, 9 skipped, 0 regressions.
• New coverage: tag pagination, semver sort with prereleases, mixed parseable/non-parseable tags, default-branch fallback, atomic-write failure cleanup, branch ref re-resolution, cache-hit skip, race-on-rename handling, path-registry-with-ref error.

Docs updated

• `docs/design/registry.md` — full rewrite of Reference syntax, Versioning, and Caching sections; updated Implementation table.
• `docs/cli-reference.md` — new syntax examples; `registry update`/`show`/`list` descriptions.
• `README.md` — quick-start example uses `#ref` form.

Notes for reviewers

• The `#` separator may need quoting in shells if surrounded by spaces (e.g. `conductor run 'qa-bot @ team # v1'`). Mid-token use (the typical case) is fine unquoted.
• Branch refs are re-resolved to SHAs on every `fetch_workflow` call, which means an extra GitHub API roundtrip when running with a branch ref. Acceptable trade-off for correctness — explicit tag refs hit the cache directly.