cloudmock-dns: rename domainConfig.Autotend → Primary, accept primary YAML key

[Viridian-Inc]

Problem

After PR #36 / #38 / #42 / #46, the only remaining brand-leak in non-test Go was `tools/cloudmock-dns/main.go`: the `domainConfig` struct field `Autotend string` and the YAML key `autotend` used to read the primary domain from the user's Pulumi config. The field name was misleading even when the value defaulted to `cloudmock.app`.

Fix

• `domainConfig.Autotend` → `domainConfig.Primary`
• All call sites (`dc.Autotend`) → `dc.Primary`
• `parsePulumiConfig` accepts `primary` as the canonical YAML key, falls back to `autotend` for backwards compat with existing autotend-infra Pulumi configs (with a comment explaining why)
• `sortedDomains` pair key updated from `"cloudmock"` (which was duplicated) to `"primary"` for the primary entry — sort still groups them deterministically
• Three new test cases:
  • `TestParsePulumiConfig_LegacyAutotendKey` — verifies the alias works
  • `TestParsePulumiConfig_PrimaryWinsOverLegacy` — verifies precedence when both keys are set
  • Existing `TestParsePulumiConfig` updated to use the new `primary` key

Out of scope

• Cosmetic comment references to `autotend` in `pkg/iac/pulumi.go` (an example of input the parser should handle in the wild) and `pkg/gateway/proxy.go` (an explanation of which historical importers the aliases serve). Both are accurate descriptive context, not aspirational coupling.
• Doc-site references in `website/src/content/docs/` and `docs/tailnet-proxy.md` (separate doc pass).

PR follows.