chore: reconcile on-client path + add golangci-lint config (R13+R14)

R13 — on-client path reconciliation:
- AGENTS.md: 6 occurrences of /opt/homelab-context/ → /opt/homelab/
  (sections 1, 2, 5, 7)
- .agents/NOMOS.md: 2 occurrences of /opt/homelab-context/ → /opt/homelab/
- CLIENTS.md already used /opt/homelab/ — now consistent across all docs.
  The repo is still named 'homelab-context' (git remote), it just clones
  to /opt/homelab/ on enrolled clients per CLIENTS.md.

R14 — golangci-lint/staticcheck/govulncheck tooling:
- .golangci.yml (new): config enabling govet, staticcheck, ineffassign,
  unused, errcheck, gosimple, typecheck, misspell, revive. Excludes
  generated code (internal/httpapi/gen/, internal/db/sqlcgen/) and
  relaxes errcheck in test files.
- Makefile: split 'lint' target into vet, golangci, govulncheck subtargets.
  Each checks if the tool is installed and prints install instructions
  if not. 'make lint' runs all three.
- CI already had golangci-lint-action + govulncheck (both advisory);
  the action auto-discovers .golangci.yml.

.agents/NOMOS.md

@@ -13,13 +13,13 @@ service itself.
 
 ## Source of truth
 
-The homelab-context repo at `/opt/homelab-context/` is the single source of
+The homelab-context repo at `/opt/homelab/` is the single source of
 truth for:
 - Fleet topology (`inventory.yaml`)
 - Agent behaviour and conventions
 - Everything in this file
 
-When in doubt, check `/opt/homelab-context/` first, or query the Oikos API/MCP
+When in doubt, check `/opt/homelab/` first, or query the Oikos API/MCP
 server directly (see [AGENTS.md](../AGENTS.md) §3-4) — the database is
 authoritative at runtime.