fix: use $HOME instead of ~ in preamble for Windows compatibility

[HMAKT99]

Summary

• replace bare ~/.gstack/ with "$HOME/.gstack/" in all preamble bash blocks
• affects marker file checks, touch, mkdir, find, and analytics appends
• update zsh-safe glob test to match new path format
• prose/documentation references left as ~/.gstack/

Problem

Every gstack skill invocation on Windows re-prompts the user to read the "Boil the Lake" essay, re-asks about telemetry consent, and re-asks about proactive mode — even after the user already answered.

The preamble checks marker files with bare ~:

_LAKE_SEEN=$([ -f ~/.gstack/.completeness-intro-seen ] && echo "yes" || echo "no")

Tilde expansion is not guaranteed in all Windows shell contexts (Git Bash subshells, MSYS2, CI runners). When ~ doesn't expand, the [ -f check always returns false and the marker is never found.

Reproduction

1. Windows 10/11 with Git Bash
2. Run any gstack skill (e.g. /office-hours)
3. Answer the Boil the Lake, telemetry, and proactive prompts
4. Run another gstack skill
5. All three prompts appear again

Root Cause

touch ~/.gstack/.completeness-intro-seen creates the file at a literal ~/.gstack/ path (or fails silently). The next invocation checks [ -f ~/.gstack/.completeness-intro-seen ] which also fails to expand ~. The marker is never written or never found.

Fix

Replace ~ with "$HOME" in all 15 executable bash paths in scripts/resolvers/preamble.ts. $HOME is set by the OS on every platform and expands reliably in all bash contexts.

Updated paths:

• mkdir -p, touch, find for sessions directory
• [ -f ... ] checks for .completeness-intro-seen, .telemetry-prompted, .proactive-prompted
• >> ... skill-usage.jsonl and eureka.jsonl appends
• rm -f ... .pending-* cleanup

Verification

bun run gen:skill-docs --dry-run
  29 FRESH, 0 STALE

bun test test/skill-validation.test.ts test/gen-skill-docs.test.ts
  563 pass, 1 fail (pre-existing: package.json version mismatch on main)

Fixes #475.