[johncwaters]

Problem

Part of #170. Kept answers live in each worktree's own .coherence/structure, so a new git worktree's first Structure reading cold-starts tsserver and asks everything again, even when another worktree of the same repo answered it all a minute ago. On my repo that's 99 s and a 1.7 GB tsserver for every new worktree, which hurts when an agent tool makes a worktree per session.

Changes

• Kept answers already hold no absolute paths (relative path, each file's sha256, a basis of Coherence's version, adapter code and config), so they move between worktrees as is.
• KeptAnswers.load() reads the worktree's own file first, then the shared one when it has none. Answers are only reused where the hashes still hold in this tree; the rest get asked.
• save() writes its own file and the shared one. A failed shared write is ignored like a failed own write.
• New shared-structure.ts: sharedStructureDir(root) (git rev-parse --git-common-dir, cached per root, undefined outside git) and writeAtomically, moved from gaps.ts so both use one.
• Scope.spec.md: new bullet "a new worktree starts from the repository's kept answers".
• No lock. The shared file is written atomically and the last writer wins. A lost write only costs some re-asking later.

How did you test this code?

• New test: a second git worktree of a read repo reads with the server started 0 times, draws the same map as a reading that asks everything, and keeps its own answers after.
• Refuted with refute (making load() read only the worktree's own file): red on the test's own assertion, then green batched and alone with run --invariant ... --each. The run records are in the PR.
• The worker-thread test now clears the shared store too, since deleting .coherence/structure alone no longer empties the answers.
• component-interfaces.test.ts and gaps.test.ts pass (19/19 with the kept-failures PR merged in too). Typecheck, lexicon:check and spec:check pass.
• On my repo (about 4,280 answers), with fresh worktrees and an empty shared store, running from source on a machine low on memory:

Run	Time	New tsserver	Answers
Cold, first worktree	259.6 s	1,650 MB	4277 asked
Fresh worktree, same commit	5.2 s	none	4280 reused
Fresh worktree, a commit no worktree had read	9.3 s	539 MB	4265 reused, 15 asked
3 fresh worktrees at once	20 to 23 s each	about 1 GB each	4265 reused, 15 asked each; shared file intact

The 3 at once each asked 15 because the shared file held the other commit's answers (last writer wins).