feat(miner): freeze/snapshot mechanism for historical replay targets

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Context

Part of the internal historical-replay calibration harness roadmap. The harness needs a way to reconstruct exactly what a real repo looked like at a chosen past commit T, without any information from after T leaking in. The discover and analyze phases of the miner pipeline already read git history to understand a target repo's shape and recent activity; that history-reading logic is the natural starting point for a freeze/snapshot exporter rather than writing a second implementation.

Goal

Given a repo and a commit T, produce a reproducible export of the working tree and a knowable-at-T-only context bundle for use as replay input.

Requirements

• Accept a repo reference and a commit SHA T; export the full working tree checked out at T via a git worktree or equivalent, without mutating the caller's working directory.
• Export a context snapshot alongside the tree: commit history up to and including T, tags/releases dated at or before T, and the README as it existed at T.
• Exclude any repo metadata dated after T (later commits, later tags, later issue/PR references) from both the tree export and the context bundle.
• Reuse the existing git-history-reading utilities from the discovery/analyze phases rather than reimplementing commit/tag enumeration.
• Persist the snapshot in a content-addressed or commit-keyed form so the same (repo, T) pair always reproduces an identical snapshot.
• Add a validation step that fails fast if any exported artifact contains a timestamp after T.

Deliverables

• Snapshot export function taking (repo, commit T) and producing a working-tree export plus a context bundle
• Post-export validation that rejects any artifact with a timestamp after T
• Reuse of existing discovery/analyze git-history utilities, documented at the call site
• Unit tests covering a repo with tags, a repo with no tags, and a commit at the very first commit of history

Expected outcomes

• A frozen snapshot for a given (repo, T) is byte-reproducible across repeated exports
• Zero post-T timestamps present in any exported artifact, verified by the validation step

Part of the historical-replay calibration roadmap: #2994

[linear-code]

JSO-2569