Extract shared install logic into lib/common.sh

[devin-ai-integration]

Summary

install.sh and uninstall.sh were near-duplicates: they shared the same config block, the remove_link_if_target helper, the GNU Stow guard, the Stow --ignore flags (each repeated twice for Claude + Codex), and the legacy-link cleanup. This factors all of that into a sourced lib/common.sh, leaving each script with only its distinct steps.

The scripts now reduce to:

source ".../lib/common.sh"
require_stow
mkdir -p "$CLAUDE_SKILLS_DIR" "$CODEX_SKILLS_DIR"
stow_skills          # install.sh
stow_skills -D       # uninstall.sh (extra flags passed through to Stow)
remove_legacy_links
# ...script-specific CLAUDE.md link / cleanup...

lib/common.sh computes REPO_DIR from ${BASH_SOURCE[0]} (not $0), so it resolves the repo root correctly whether sourced from either script.

Two behavioral fixes fall out of the consolidation:

• The shared STOW_IGNORES list adds --ignore='lib', so the new helper directory is not itself symlinked into ~/.claude/skills / ~/.codex/skills.
• The legacy-skill cleanup is now a single LEGACY_SKILLS array used by both scripts. Previously uninstall.sh was missing the grill-me entry that install.sh had, so a stale grill-me link could survive an uninstall; both scripts now stay in sync.

Verification

• shellcheck install.sh uninstall.sh lib/common.sh — clean.
• Ran ./install.sh against temp CLAUDE_HOME/CODEX_HOME: all skills symlinked; lib, README.md, AGENTS.md, install.sh, uninstall.sh correctly excluded; CLAUDE.md linked to AGENTS.md. Re-running is idempotent.
• Ran ./uninstall.sh: all skill links, CLAUDE.md, and the now-empty skills dirs removed.

Link to Devin session: https://app.devin.ai/sessions/62a9f4fade1c4277bc0e272137e1a8d9
Requested by: @sidwood

[devin-ai-integration]

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