refactor(git-read): use shared search seams

[seekskyworld]

Problem

Implements the cross-extension import slice of #73. git-read directly imported private file-search/src modules, coupling extension internals.

Value

A named shared seam makes the dependency direction explicit and gives future refactors one stable import boundary without changing command, output, or process semantics.

Approach

Add shared re-export seams for bounded search output and process execution, then update git-read to consume only those shared modules. The underlying implementation and behavior remain unchanged.

Validation

• npx tsc --noEmit
• git diff --check

Impact

• User-visible behavior: none.
• Model-visible context/tools: none.
• Runtime/lifecycle: no behavior change.
• Persisted config/data: none.
• Compatibility/risk: additive shared modules; remaining 工程与发布契约：消除跨扩展内部依赖、测试发现盲区和版本声明漂移 #73 items are intentionally out of scope.

[tt-a1i]

Exact-head review: a9d7ec4

[P1] This moves the import name but does not remove the cross-extension private dependency required by Issue #73. extensions/shared/search-output.ts and search-process.ts are only re-export shims whose implementations still come from ../file-search/src/output.ts and ../file-search/src/process.ts. The shared layer therefore depends on a feature extension private src tree; refactoring or packaging file-search can still break git-read through the new alias.

Issue #73 acceptance requires extensions to stop importing another extension private implementation, and the PR claims a stable shared seam. Please move the reusable bounded-output/process implementation into extensions/shared (or define a genuinely owner-neutral shared module), then have both file-search and git-read depend on it. Add an import-boundary guard so a future shared-to-feature dependency fails CI.

Verification boundary: exact-head Standards + Spec static review; no runtime behavior change is claimed, so the blocker is the unresolved dependency direction itself.