chore(security): install global security baseline + project rules P011-P016

[shyhunter]

Summary

Establishes the project's surface of the workspace-wide security baseline (R005–R016 in ~/.claude/rules.md).

• .gitignore — env-file coverage with example file allowed (R006)
• .claude/project_rules_decisions.md — adds P011–P016 covering Tauri capabilities, sidecar provenance, updater key handling, renderer trust boundary, npm audit + gitleaks in CI
• .claude/CLAUDE.md — pointer to the inherited baseline
• src-tauri/binaries/README.md — documents Ghostscript sidecar source URL + SHA-256 (R011 supply-chain pinning)
• .github/workflows/security.yml — gitleaks on every push/PR + npm audit --omit=dev --audit-level=high (R006, R011)
• .gitleaks.toml — useDefault = true with project-specific path allowlist for test fixtures and redaction-library examples (drops 51 false-positives to 0)

Test plan

• gitleaks scan locally: 0 findings after allowlist
• npm audit --omit=dev will be exercised by the new workflow on first push
• Reviewer: confirm no path-allowlist entry inadvertently hides a real secret

Security notes (R015)

• Secrets: none introduced; the change adds secret-scanning + env-file ignores.
• Auth/permissions: workflow runs with permissions: contents: read; uses ${{ secrets.GITHUB_TOKEN }} only.
• Remaining risks: the .gitleaks.toml allowlist is path-based — any new file added under those paths is implicitly trusted. If sensitive content is added under test-fixtures/ or src/lib/secrets/__tests__/, gitleaks will not flag it. Mitigation: code review owns those paths.

🤖 Generated with Claude Code