Add shell command explainer (#75004)

Summary:
- The PR adds an internal Tree-sitter-backed shell command explainer under `src/infra`, parser runtime/tests, dependency/build-policy updates, an index export, and a changelog entry.
- Reproducibility: not applicable. this is a feature PR rather than a bug report. For the prior PR blocker, source inspection shows byte-to-string span conversion and focused Unicode span coverage on the exact head.

Automerge notes:
- Ran the ClawSweeper repair loop before final review.
- Included post-review commit in the final squash: Repair shell command explainer automerge blockers
- Included post-review commit in the final squash: fix(clawsweeper): address review for automerge-openclaw-openclaw-7500…

Validation:
- ClawSweeper review passed for head 47577579e9.
- Required merge gates passed before the squash merge.

Prepared head SHA: 47577579e9
Review: https://github.com/openclaw/openclaw/pull/75004#issuecomment-4351322592

Co-authored-by: Jesse Merhi <jessejmerhi@gmail.com>
Co-authored-by: clawsweeper <274271284+clawsweeper[bot]@users.noreply.github.com>

pnpm-workspace.yaml

@@ -48,3 +48,4 @@ onlyBuiltDependencies:
 
 ignoredBuiltDependencies:
   - koffi
+  - tree-sitter-bash