fix: use $HOME instead of ~ in installed shell command paths (#1284)

The installer pathPrefix for global installs replaced os.homedir()
with ~, which does NOT expand inside double-quoted shell commands
in POSIX shells. This caused MODULE_NOT_FOUND errors when executing
commands like: node "~/.claude/get-shit-done/bin/gsd-tools.cjs"

Changed pathPrefix to use $HOME instead of ~, which correctly expands
inside double quotes. Also fixed a quoted-tilde instance in do.md.

- bin/install.js: $HOME prefix instead of ~ for global installs
- get-shit-done/workflows/do.md: node "$HOME/..." instead of "~/"
- tests/path-replacement.test.cjs: updated + 3 new regression tests

Closes #1284

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

get-shit-done/workflows/do.md

@@ -24,7 +24,7 @@ Wait for response before continuing.
 **Check if project exists.**
 
 ```bash
-INIT=$(node "~/.claude/get-shit-done/bin/gsd-tools.cjs" state load 2>/dev/null)
+INIT=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" state load 2>/dev/null)
 ```
 
 Track whether `.planning/` exists — some routes require it, others don't.