chore: remove Trellis tooling and patch undici

[leonfox28]

Remove the repository-managed Trellis workflow, skills, agent hooks, task archives, journals, and journal merge rule. Preserve all 17 backend, frontend, and infrastructure guides under docs/engineering/, update their cross-references, and point AGENTS.md and the documentation map at the canonical project documents.

Patch jsdom's transitive undici dependency from 7.29.0 to 7.29.1 to resolve the dependency audit failures. Use the existing workspace override mechanism for affected 7.x versions and regenerate the lockfile with the pinned pnpm version.

Validation:

• No remaining Trellis paths/content references in tracked or unignored files or ignored project configuration; every migrated guide was compared with its previous content.
• make check-docs: 59 Markdown files passed.
• Frozen lockfile installation, 71 Web tests, TypeScript checking, production Web build, and both desktop/mobile Playwright regressions passed.
• make verify-generated passed without generated-file changes.
• Production and full pnpm audit --audit-level moderate both reported no known vulnerabilities.
• Gitleaks v8.30.1 scanned the complete intended working tree without leaks; diff whitespace checks passed.
• CI and container validation passed, including all three production image builds and supported Linux cross-builds.

The Web build retains its nonfatal warning for an existing chunk over 500 kB.