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/cli.py

@@ -140,6 +140,17 @@ def _cmd_status(emit_json: bool) -> int:
         disabled = info.get("disabled_servers") or []
         if disabled:
             out.append(f"  disabled in cfg: {', '.join(disabled)}")
+
+    # Surface backend-tool gaps that aren't visible in the registry table:
+    # some servers spawn fine but emit no diagnostics without a sidecar
+    # binary (bash-language-server -> shellcheck).
+    backend_warnings = _backend_warnings()
+    if backend_warnings:
+        out.append("")
+        out.append("Backend warnings")
+        out.append("================")
+        for line in backend_warnings:
+            out.append(f"  ! {line}")
     out.append("")
     out.append("Registered Servers")
     out.append("==================")
@@ -268,3 +279,30 @@ def _recipe_pkg_for(server_id: str) -> str:
         "typescript": "typescript-language-server",
     }
     return aliases.get(server_id, server_id)
+
+
+def _backend_warnings() -> list:
+    """Return human-readable notes about LSP backend tools that are missing
+    in a way that won't surface elsewhere.
+
+    Some language servers ship as thin wrappers around an external CLI for
+    actual diagnostics — they spawn cleanly but never emit any errors when
+    the sidecar binary isn't on PATH.  bash-language-server / shellcheck
+    is the load-bearing example.
+
+    Returned strings are short, actionable, and include the install
+    suggestion across common platforms.
+    """
+    import shutil as _shutil
+    from agent.lsp.install import hermes_lsp_bin_dir
+    notes: list = []
+    bash_installed = _shutil.which("bash-language-server") is not None or (
+        (hermes_lsp_bin_dir() / "bash-language-server").exists()
+    )
+    if bash_installed and _shutil.which("shellcheck") is None:
+        notes.append(
+            "bash-language-server is installed but shellcheck is missing — "
+            "diagnostics will be empty (apt: shellcheck, brew: shellcheck, "
+            "scoop: shellcheck)."
+        )
+    return notes