chore: track Nx 22 AI tooling artefacts injected by generators

Nx 22 generators inject AI agent tooling into the repo via marked
sections and side files. Rather than re-deleting them after every
generator run, track the workspace-level ones and document why.

- CLAUDE.md gains a 'General Guidelines for working with Nx' section
  between Nx-managed markers; future Nx versions will update this
  section automatically without touching our project rules above.
  Pre-existing prettier-formatted blank lines added before code blocks
  are kept.
- .claude/settings.json (228 bytes) enables the nx-claude-plugins
  marketplace from nrwl/nx-ai-agents-config; tracked for contributor
  consistency. The personal .claude/settings.local.json stays
  gitignored.
- .github/ is the Nx AI skills/prompts/agents catalog. Kept despite
  being GitHub-named; it does not conflict with our Gitea workflows
  which will live under .gitea/workflows/ (per ADR-0015).
- .gitignore picks up Nx-managed transient dirs (.nx/polygraph,
  .claude/worktrees) and a trailing newline fix.

AGENTS.md is removed: it duplicated only the Nx auto-injected guidance
that CLAUDE.md already carries (CLAUDE.md is the strictly broader file
- project rules + Nx section). One source of truth for AI-agent
guidance.

.github/skills/nx-workspace/references/AFFECTED.md

@@ -0,0 +1,27 @@
+## Affected Projects
+
+Find projects affected by changes in the current branch.
+
+```bash
+# Affected since base branch (auto-detected)
+nx show projects --affected
+
+# Affected with explicit base
+nx show projects --affected --base=main
+nx show projects --affected --base=origin/main
+
+# Affected between two commits
+nx show projects --affected --base=abc123 --head=def456
+
+# Affected apps only
+nx show projects --affected --type app
+
+# Affected excluding e2e projects
+nx show projects --affected --exclude="*-e2e"
+
+# Affected by uncommitted changes
+nx show projects --affected --uncommitted
+
+# Affected by untracked files
+nx show projects --affected --untracked
+```