fix(lsp): typescript SDK install + tsc-missing skip + shellcheck warning (#24630)

Three follow-ups to PR #24168 found during live E2E testing on TS/bash files:

1. typescript-language-server now installs the typescript SDK (tsserver)
   alongside it. Without that sibling install, initialize() failed with
   "Could not find a valid TypeScript installation" and the server was
   marked broken — no diagnostics ever reached the agent. New extra_pkgs
   field on INSTALL_RECIPES makes that explicit and reusable for future
   peer-dep cases.

2. _check_lint now treats "linter command exists on PATH but cannot
   actually run" as skipped instead of error. The motivating case is
   npx tsc when typescript is not in node_modules — npx prints its
   "This is not the tsc command you are looking for" banner and exits
   non-zero, which previously blocked the LSP semantic tier (gated on
   success or skipped). Pattern-matched per base command (npx,
   rustfmt, go) so genuine lint errors still flow through normally.

3. hermes lsp status now surfaces a Backend warnings section when
   bash-language-server is installed but shellcheck is missing. The
   server itself spawns fine but bash-language-server delegates
   diagnostics to shellcheck — without it on PATH the integration
   looks alive but never reports any problems. Same warning is
   logged once at server spawn time.

Validation:

- 12 new tests in tests/agent/lsp/test_install_and_lint_fixes.py:
    * recipe carries typescript SDK
    * _install_npm passes both pkg + extras to npm CLI
    * backwards compat: recipes without extras still work
    * _backend_warnings quiet when bash absent / both present
    * _backend_warnings fires when bash installed without shellcheck
    * status output includes the Backend warnings section
    * _looks_like_linter_unusable catches the npx tsc banner
    * real TS type errors not misclassified as unusable
    * unfamiliar linters fall through normally
    * _check_lint returns skipped on npx tsc unusable
    * _check_lint returns error on real tsc type errors
- Full lsp + file_operations test suite: 245/245 pass
- Live E2E:
    * try_install("typescript-language-server") installs both packages
      into node_modules
    * write_file(bad.ts, ...) returns lint=skipped + lsp_diagnostics
      with two real TS errors (was lint=error, no lsp_diagnostics)
    * hermes lsp status renders the shellcheck warning when bash is
      installed but shellcheck is not on PATH

agent/lsp/install.py

@@ -33,7 +33,7 @@ import subprocess
 import sys
 import threading
 from pathlib import Path
-from typing import Dict, Optional
+from typing import Any, Dict, Optional
 
 logger = logging.getLogger("agent.lsp.install")
 
@@ -41,7 +41,13 @@ logger = logging.getLogger("agent.lsp.install")
 # tuple of strategy name + package name + executable name.  When the
 # install completes, we look for the executable in
 # ``<HERMES_HOME>/lsp/bin/`` first, then on PATH.
-INSTALL_RECIPES: Dict[str, Dict[str, str]] = {
+#
+# Optional fields:
+#   - ``extra_pkgs``: list of sibling packages to install alongside
+#     ``pkg`` in the same node_modules tree.  Used when an LSP server
+#     has a runtime peer dependency that npm doesn't auto-pull (e.g.
+#     typescript-language-server needs ``typescript``).
+INSTALL_RECIPES: Dict[str, Dict[str, Any]] = {
     # Python
     "pyright": {"strategy": "npm", "pkg": "pyright", "bin": "pyright-langserver"},
     # JS/TS family
@@ -49,6 +55,11 @@ INSTALL_RECIPES: Dict[str, Dict[str, str]] = {
         "strategy": "npm",
         "pkg": "typescript-language-server",
         "bin": "typescript-language-server",
+        # typescript-language-server requires the `typescript` SDK
+        # (tsserver) to be importable from the same node_modules tree;
+        # otherwise initialize() fails with "Could not find a valid
+        # TypeScript installation".  Install them together.
+        "extra_pkgs": ["typescript"],
     },
     "@vue/language-server": {
         "strategy": "npm",
@@ -179,7 +190,11 @@ def _do_install(pkg: str) -> Optional[str]:
         return None
 
     if strategy == "npm":
-        return _install_npm(recipe.get("pkg", pkg), bin_name)
+        return _install_npm(
+            recipe.get("pkg", pkg),
+            bin_name,
+            extra_pkgs=recipe.get("extra_pkgs") or [],
+        )
     if strategy == "go":
         return _install_go(recipe.get("pkg", pkg), bin_name)
     if strategy == "pip":
@@ -189,22 +204,36 @@ def _do_install(pkg: str) -> Optional[str]:
     return None
 
 
-def _install_npm(pkg: str, bin_name: str) -> Optional[str]:
+def _install_npm(
+    pkg: str,
+    bin_name: str,
+    extra_pkgs: Optional[list] = None,
+) -> Optional[str]:
     """Install an npm package into our staging dir.
 
     Uses ``npm install --prefix`` so the binaries land in
     ``<staging>/node_modules/.bin/<bin_name>`` and we symlink them up
     one level for direct PATH-style access.
+
+    ``extra_pkgs`` is a list of sibling packages to install in the
+    same ``node_modules`` tree.  Used for LSP servers with runtime
+    peer deps that npm doesn't auto-pull (typescript-language-server
+    needs ``typescript`` next to it; intelephense ships standalone).
     """
     npm = shutil.which("npm")
     if npm is None:
         logger.info("[install] cannot install %s: npm not on PATH", pkg)
         return None
     staging = hermes_lsp_bin_dir().parent  # <HERMES_HOME>/lsp/
+    install_targets = [pkg] + list(extra_pkgs or [])
     try:
-        logger.info("[install] npm install --prefix %s %s", staging, pkg)
+        logger.info(
+            "[install] npm install --prefix %s %s",
+            staging,
+            " ".join(install_targets),
+        )
         proc = subprocess.run(
-            [npm, "install", "--prefix", str(staging), "--silent", "--no-fund", "--no-audit", pkg],
+            [npm, "install", "--prefix", str(staging), "--silent", "--no-fund", "--no-audit", *install_targets],
             check=False,
             capture_output=True,
             text=True,