fix(gstack-paths): shell-quote output so eval is safe for paths with spaces

[yannickspiess]

Problem

GSTACK_HOME (and the other path roots) may contain spaces or shell metacharacters — for example an iCloud-synced directory under Library/Mobile Documents/.

The current bare echo emits unquoted output:

GSTACK_STATE_ROOT=/Users/me/Library/Mobile Documents/Obsidian/GStack

When skills do eval "$(gstack-paths)", bash/zsh split on the first space:

• GSTACK_STATE_ROOT gets set to /Users/me/Library/Mobile
• The shell then tries to execute Documents/Obsidian/GStack as a command

Result: command not found and GSTACK_STATE_ROOT left truncated/wrong.

Fix

Add a POSIX-compatible _shell_quote helper that wraps each value in single quotes and properly escapes any embedded single quotes (the standard '\''\'' form). Replace the three echo lines with printf.

Output now looks like:

GSTACK_STATE_ROOT='/Users/me/Library/Mobile Documents/Obsidian/GStack'
PLAN_ROOT='/Users/me/.claude/plans'
TMP_ROOT='/var/folders/...'

Safe for paths containing: spaces, $, ;, backticks, double quotes, single quotes.

Works in sh / bash / zsh / dash (no printf %q which is bash/zsh-only).

Testing

Verified with:

• Normal path (no spaces) — behaviour unchanged
• Path with spaces — now works
• Actual iCloud vault path (Library/Mobile Documents/iCloud~md~obsidian/...) — now works

Notes

The existing comment said "output values are not sanitized" — that warning is now removed since the output is sanitized. No other call sites need changing; the output contract (eval "$(gstack-paths)") is preserved.