Phase C: Automated TLS cert provisioning for L7 routes

Add CertProvisioner that requests TLS certificates from Metacrypt's CA
API during deploy. When a service has L7 routes, the agent checks for
an existing cert, re-issues if missing or within 30 days of expiry,
and writes chain+key to mc-proxy's cert directory before registering
routes.

- Add MetacryptConfig to agent config (server_url, ca_cert, mount,
  issuer, token_path) with defaults and env overrides
- Add CertProvisioner (internal/agent/certs.go): REST client for
  Metacrypt IssueCert, atomic file writes, cert expiry checking
- Wire into Agent struct and deploy flow (before route registration)
- Add hasL7Routes/l7Hostnames helpers in deploy.go
- Fix pre-existing lint issues: unreachable code in portalloc.go,
  gofmt in servicedef.go, gosec suppressions, golangci v2 config
- Update vendored mc-proxy to fix protobuf init panic
- 10 new tests, make all passes with 0 issues

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modules.txt

@@ -1,4 +1,4 @@
-# git.wntrmute.dev/mc/mc-proxy v1.1.0
+# git.wntrmute.dev/mc/mc-proxy v1.1.0 => /home/kyle/src/metacircular/mc-proxy
 ## explicit; go 1.25.7
 git.wntrmute.dev/mc/mc-proxy/client/mcproxy
 git.wntrmute.dev/mc/mc-proxy/gen/mc_proxy/v1
@@ -192,3 +192,4 @@ modernc.org/memory
 modernc.org/sqlite
 modernc.org/sqlite/lib
 modernc.org/sqlite/vtab
+# git.wntrmute.dev/mc/mc-proxy => /home/kyle/src/metacircular/mc-proxy