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/servers.py

@@ -336,6 +336,9 @@ def _spawn_clangd(root: str, ctx: ServerContext) -> Optional[SpawnSpec]:
     )
 
 
+_BASH_SHELLCHECK_WARNED = False
+
+
 def _spawn_bash_ls(root: str, ctx: ServerContext) -> Optional[SpawnSpec]:
     bin_path = _resolve_override(ctx, "bash-language-server") or _which("bash-language-server")
     if bin_path is None:
@@ -343,6 +346,18 @@ def _spawn_bash_ls(root: str, ctx: ServerContext) -> Optional[SpawnSpec]:
         bin_path = try_install("bash-language-server", ctx.install_strategy)
         if bin_path is None:
             return None
+    # bash-language-server delegates diagnostics to ``shellcheck``.  Without
+    # it on PATH the server starts and accepts requests but never reports
+    # any problems — to the user it looks like a working integration that
+    # never finds bugs.  Warn once so the gap is visible.
+    global _BASH_SHELLCHECK_WARNED
+    if not _BASH_SHELLCHECK_WARNED and _which("shellcheck") is None:
+        _BASH_SHELLCHECK_WARNED = True
+        logger.warning(
+            "bash-language-server: shellcheck not found on PATH — "
+            "diagnostics will be empty until shellcheck is installed "
+            "(apt: shellcheck, brew: shellcheck, scoop: shellcheck)."
+        )
     return SpawnSpec(
         command=[bin_path, "start"],
         workspace_root=root,