refactor(core): centralize boundary normalization

[kitlangton]

What

Centralize repeated normalization at ripgrep, MCP, and provider boundaries. Each helper exactly preserves the previous path, error, and record semantics while removing local duplication and assertions.

How

• Reuse one path normalizer across ripgrep glob, find, and grep results.
• Reuse one unknown-to-Error conversion across seven MCP promise boundaries.
• Use the canonical AI package isRecord guard when merging provider overlays.

Scope

Behavior-preserving Core cleanup only; no path policy, MCP recovery, provider merge, or public API changes.

Testing

• bun run test test/ripgrep.test.ts test/mcp.test.ts test/provider.test.ts (38 passed)
• bun typecheck from packages/core
• Push hook: bun turbo typecheck --concurrency=3 (40-package workspace)

[kitlangton]

Consolidated into #43929 so the cleanup ships as one coherent persistence and boundary-normalization review.