docs: add client lifecycle plan, cleanup stale files, document repo for 3 audiences

Problem: Repo had no developer guide, no client onboarding doc, no agent
dev instructions. Stale files (675KB SQL dump, one-off convert script,
legacy MCP builder) cluttered the tree. Client enrollment was a documented
intention with no Go implementation.

Changes:
- New docs: CONTRIBUTING.md (dev setup), CLIENTS.md (client onboarding),
  .agents/dev/CONTRIBUTING.md (agent codebase map)
- New plan: plans/2026-07-07-client-lifecycle-in-go.md — full client
  lifecycle (planned→provisioning→active→deprecated→destroyed) in Go,
  replacing archived Python secrets-issuance, adding client API endpoints
  and 6 missing MCP tools
- Cleanup: deleted archive/convert-wiki.py (one-off), archive/mcp/
  build_host_files.py (legacy), backups/pre-deploy-7f7d039.sql (local)
- Fixes: plans/index.md duplicate row removed, README.md repo layout
  updated for current state, AGENTS.md header points to new guides

Risk: low. Docs only + stale file deletion. No code changes. New plan is
proposal, not implementation.
Verification: git diff reviewed, all changes are prose/docs/plans.

plans/index.md

@@ -10,10 +10,10 @@ went sideways, open an investigation.
 | ---- | ----- | ------ |
 | 2026-07-05 | [Oikos Prometheus LXC](2026-07-05-oikos-prometheus-lxc.md) | Planned |
 | 2026-07-06 | [Consolidate Oikos control plane onto mac-mini](2026-07-06-consolidate-oikos-control-plane-onto-mac-mini.md) | In Progress (Phase 1-6 implemented, pending cutover) |
+| 2026-07-07 | [Client lifecycle in Go — enrollment through deprecation](2026-07-07-client-lifecycle-in-go.md) | Planned |
 | 2026-07-07 | [Comprehensive audit: stale files, state gaps, and next steps](2026-07-07-comprehensive-audit-and-next-steps.md) | Planned |
 | 2026-07-07 | [DB as single source of truth for agent knowledge](2026-07-07-db-as-source-of-truth.md) | Proposed |
 | 2026-07-07 | [Migrate bin/homelab CLI to Go oikos homelab](2026-07-07-migrate-bin-homelab-to-go.md) | Planned |
-| 2026-07-07 | [Migrate bin/homelab CLI to Go oikos homelab](2026-07-07-migrate-bin-homelab-to-go.md) | Planned |
 
 ## Done