grep: restore as an eval-only builtin backed by an in-process native engine with six-host prebuilds

## Summary
Restore `grep` as an eval-only builtin backed by an in-process native engine, with six-host prebuilds for supported release artifacts.

## Reproduction
In an eval cell, `tool.grep` returns `Unknown tool grep`; `tool_schema("grep")` returns not found.

## Expected
Ideal state = Scope Must have, sanitized: eval can call `tool.grep` with a stable schema, the builtin remains eval-only, the native engine is in-process, and release artifacts provide the required host prebuilds.

## Actual
The grep builtin is withheld from the exposed tool set, so eval callers cannot discover or invoke it.

## Evidence
- `packages/coding-agent/src/core/tools/index.ts:107-114`
- `packages/coding-agent/src/core/agent-session.ts:7548-7551,7612-7618`
- PR #1186

## Root cause
The current exposure and workflow guidance intentionally withhold grep from eval, leaving no native eval-only implementation or packaging path.

## Scope
Must have:
- expose grep only to eval with a stable schema and direct-call guidance;
- add an in-process native engine with an engine-agnostic layer;
- add six-host prebuild packaging and release verification;
- preserve existing permission, output, pagination, and cancellation contracts.

Acceptance criteria:
- eval discovery and invocation succeed while non-eval surfaces remain unchanged;
- native and fallback engines pass parity tests for matching, limits, errors, and cancellation;
- release artifacts contain the required host prebuilds and package verification passes;
- documentation and CI checks cover the contract.

## Related
- #1186 (withhold grep and reframe the eval graph section as workflow)
- #1192 (bashEvalOnly)
- #1198 (workflowEvalOnly)

Start with packages/coding-agent/src/core/tools/index.ts:107-114 and packages/coding-agent/src/core/agent-session.ts:7548-7551,7612-7618 to understand current grep exposure and eval workflow, then review PR #1186 and the related eval-only issues. Done means eval discovery and invocation work without changing non-eval surfaces, native and fallback engines meet the stated parity contracts, six-host release artifacts verify successfully, and documentation and CI checks cover the contract.

Written by the indexing model from the issue text.