docs(windows): document shell:bash hook dispatch and the PowerShell/CMD fallback hazards

The SessionStart command string starts with a quoted path, which breaks
both Windows shells Claude Code may hand it to: PowerShell parses the
leading quoted string as an expression and dies on the next bareword
('Unexpected token session-start', #1751), and cmd.exe's /c quote rule
drops the outer quotes when the path contains a metacharacter, so a
profile dir like C:\Users\Name(External) truncates the command at the
'(' (#1918). Either way the bootstrap silently never loads.

Declare shell: "bash" on the hook. Claude Code >= 2.1.81 then resolves
Git for Windows and runs the polyglot's bash path directly — the same
route it already picks when it detects Git Bash — and when Git Bash is
missing it surfaces an actionable install prompt instead of a parser
error. Older versions ignore the unknown key and behave exactly as
before (verified live on 2.0.77 and 2.1.80).

Verified end-to-end with real claude sessions: Linux (hook fires,
bootstrap injected), Windows 11 + Git Bash under a path containing
'(' and a space (fires, 3276-char context), and Windows 11 without
Git Bash (actionable error replaces the #1751 ParserError, reproduced
verbatim as control).

Fixes #1751
Fixes #1918

docs/windows/polyglot-hooks.md

@@ -6,9 +6,18 @@ Claude Code plugins need hooks that work on Windows, macOS, and Linux. This docu
 
 ## The Problem
 
-Claude Code runs hook commands through the system's default shell:
-- **Windows**: CMD.exe
+Claude Code runs hook commands through a shell:
 - **macOS/Linux**: bash or sh
+- **Windows with Git Bash installed**: Git Bash
+- **Windows without Git Bash**: PowerShell (older versions used CMD.exe)
+
+Neither Windows fallback shell can parse our command string: PowerShell treats
+a leading quoted path as a string expression and errors on the next bareword,
+and CMD.exe's `/c` quoting rules strip the outer quotes when the path contains
+a metacharacter such as `(`. Our hooks therefore declare `"shell": "bash"`
+(supported since Claude Code 2.1.81; older versions ignore the key), which
+forces the Git Bash route and, when Git Bash is absent, produces an actionable
+"install Git for Windows" error instead of a shell parser failure.
 
 This creates several challenges:
 
@@ -42,6 +51,7 @@ hooks/
           {
             "type": "command",
             "command": "\"${CLAUDE_PLUGIN_ROOT}/hooks/run-hook.cmd\" session-start",
+            "shell": "bash",
             "async": false
           }
         ]

hooks/hooks.json

@@ -7,6 +7,7 @@
           {
             "type": "command",
             "command": "\"${CLAUDE_PLUGIN_ROOT}/hooks/run-hook.cmd\" session-start",
+            "shell": "bash",
             "async": false
           }
         ]

tests/hooks/test-session-start.sh

@@ -143,6 +143,27 @@ for (const forbiddenText of forbiddenTexts) {
 
 echo "SessionStart hook output tests"
 
+# Registration shape: the hook must declare shell:"bash" so Claude Code on
+# Windows dispatches via Git Bash (or fails with an actionable error) instead
+# of PowerShell/cmd.exe, whose parsers break on the quoted command string
+# (PowerShell ParserError; cmd.exe quote-stripping on paths with metacharacters).
+if node -e '
+const hooks = JSON.parse(require("fs").readFileSync(process.argv[1], "utf8"));
+const entry = hooks.hooks.SessionStart[0].hooks[0];
+if (entry.shell !== "bash") {
+  console.error(`SessionStart hook shell is ${JSON.stringify(entry.shell)}, expected "bash"`);
+  process.exit(1);
+}
+if (!/run-hook\.cmd" session-start$/.test(entry.command)) {
+  console.error(`unexpected SessionStart command shape: ${entry.command}`);
+  process.exit(1);
+}
+' "$REPO_ROOT/hooks/hooks.json"; then
+    pass "hooks.json registers SessionStart with shell:bash dispatch"
+else
+    fail "hooks.json registers SessionStart with shell:bash dispatch"
+fi
+
 claude_home="$(make_home claude-code)"
 assert_command_output \
     "Claude Code emits nested SessionStart additionalContext" \