feat: bootstrap scaffold sync base from Cella-Base trailer with tree-similarity fallback

[flipvh]

Summary

A fresh create-cella scaffold shares no git history with upstream, so the first cella sync had no merge-base and failed with "no common ancestor" unless the user manually seeded refs/cella/last-sync. The old mechanism for this — a committed .cella/base file written by create-cella — was never read by this CLI.

ensureSyncBase now resolves the bootstrap base with one uniform rule: try each source in order; the first one whose commit actually exists after the upstream fetch wins.

1. refs/cella/last-sync (sync-point record, unchanged)
2. cella.manifest.json committed at HEAD, then the worktree copy
3. New: the root commit's Cella-Base: <sha> trailer, stamped by create-cella (see companion PR feat: stamp scaffold provenance as a Cella-Base trailer, drop .cella/base create-cella#12)
4. New: tree-similarity inference — score the fork's root snapshot against recent upstream commits (diff-tree --name-only, up to 400 candidates no newer than the root commit) and take the closest match, rejected when >30% of paths differ

The trailer travels with push/clone (unlike a local ref) and needs no worktree file. Inference reseeds scaffolds whose trailer is absent or stale: local-template scaffolds, amended/squashed root commits, upstream history rewrites. Whichever source wins is persisted (ref immediately, manifest on the first applied sync), so this runs at most once per fork lifetime.

Behavior change: a recorded base that is missing after the fetch (stale manifest, foreign upstream) now falls through to the next source instead of hard-failing.

Tests

• New e2e suite tests/e2e/ensure-sync-base.test.ts builds scaffold-shaped fixture repos (snapshot + template-cleaner-style edits + unrelated history) and covers: exact inference with newer upstream commits and fork-local work present, tie-breaking toward the newest candidate, trailer priority over inference, fall-through on a bogus trailer SHA, and rejection of a non-scaffold repo.
• Full suite: 124 tests pass; verified end-to-end against a real scaffold of the actual template (~2000 files, bootstrap in well under 6s).

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