chore: add sethLabels deployment-fork scaffold

Adds Seth's homelab convention files on top of upstream glabels-qt master:
CLAUDE.md (project instructions), IDEA.md (brief), DECISIONS.md (decision
log, empty), and the kickoff handoff under .claude/handoffs/. Appends a
clearly-marked sethLabels section to .gitignore for .backup/, the
GITEA_API.md symlink, .env files, and handoff drafts.

This is the only non-upstream commit on main. All preceding history is
authored by the glabels.org team (Jaye Evins et al.) and untouched.
Future upstream merges should fast-forward or rebase cleanly since the
scaffold lives in distinct files at repo root.

No source-code changes. License unchanged (GPL-3.0).

DECISIONS.md

@@ -0,0 +1,12 @@
+# DECISIONS.md — sethLabels Decision Log
+
+Project-specific decisions. For global/cross-cutting decisions, see `~/bin/DECISIONS.md`.
+
+Format: `YYYY-MM-DD: <decision> — <why>`
+
+## Architecture
+
+## Implementation
+
+## Deferred / Rejected
+<!-- Decisions NOT to do something are just as valuable -- prevents re-proposing rejected ideas -->