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.

AGENTS.md

@@ -4,6 +4,9 @@ You are running on a machine that is part of the **hubris** homelab. The full
 context is in this checkout at `/opt/homelab-context/`. This file is the entry
 point. Read it once at start, then keep working.
 
+- **New client?** Read [CLIENTS.md](CLIENTS.md) first.
+- **Developing on this repo?** Also read [.agents/dev/CONTRIBUTING.md](.agents/dev/CONTRIBUTING.md).
+
 The operating model — OODA loop, risk classes, approval rules, the ontology,
 and node lifecycle — is defined in [OIKOS.md](.agents/OIKOS.md). Before any mutation,
 classify the action against `seeds/policy.yaml`; when the class requires