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>

package.json

@@ -1700,10 +1700,12 @@
     "sqlite-vec": "0.1.9",
     "tar": "7.5.13",
     "tokenjuice": "0.7.0",
+    "tree-sitter-bash": "^0.25.1",
     "tslog": "^4.10.2",
     "typebox": "1.1.37",
     "undici": "8.1.0",
     "web-push": "^3.6.7",
+    "web-tree-sitter": "^0.26.8",
     "ws": "^8.20.0",
     "yaml": "^2.8.3",
     "zod": "^4.4.1"
@@ -1784,7 +1786,8 @@
       "sharp"
     ],
     "ignoredBuiltDependencies": [
-      "koffi"
+      "koffi",
+      "tree-sitter-bash"
     ],
     "packageExtensions": {
       "@mariozechner/pi-coding-agent": {