feat: git history visualization mode

[tcsenpai]

Replay a repository's commit history on the code city — one commit per event, oldest→newest — so the repo grows from nothing as it plays. Builds on the map/video work that just merged (#1); it's another trace source through the same pipeline.

What it does

• New internal/githistory package builds a trace from git log --numstat, reusing the existing trace / playback / citymap / ghost pipeline unchanged. Each commit → one event with an edit target per changed file; per-file churn (added+deleted) rides on the target. History is capped via --max-count to bound memory on large repos.
• Empty-repo seed event at t=0 so the first commit reads as the first change, not the starting frame.
• mindwalk history <repo> CLI, ?history=1 UI mode, and GET /api/history[?repo=<path>] emitting the full {trace, city} snapshot. Cached per repo with a TTL + bounded eviction; the git log + citymap build runs outside the cache lock so concurrent requests for other repos aren't serialized behind it (mirrors the existing inflight intent).
• Reachable from a running serve UI via the HUD Explore menu and the sidebar "Explore a repository" picker — each offering Map and History.
• Terrain colors in history mode: each file colored by the size (churn) of the commit that last touched it on the LOC ramp (grey→red) against a fixed ceiling; files touched by the commit at the playhead glow blue to mark whose turn it is.

Notes

• Deleted-over-history files render as ghosts (existing mechanism).
• No changes to the model, schema, playback engine, or the session / static-map paths — history is just another trace source.
• Tests: internal/githistory (replay ordering, rename→new-path, delete→ghost, numstat churn, seed event, non-repo error) and internal/server (history endpoint, 404 without repo, cache TTL + eviction bound). make test passes; gofmt-clean.
• Embedded assets regenerated via make build.