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.