docs: correct contributor prerequisites and stale dev-env references

[rdimitrov]

The README prerequisites were both stale, and the docs restated the Go version in three places.

Prerequisites: name the source, do not restate the version

README said	Reality
Go 1.24.x	go 1.26 in go.mod
golangci-lint v2.4.0	v2.11.4 pinned in ci.yml:30

Rather than swap in fresher numbers — which only resets the clock on the same drift — this removes the numbers and names the authoritative file:

- **Go** - version per the `go` directive in go.mod; the Go toolchain fetches it for you
- **golangci-lint** - version per the `golangci-lint-action` pin in ci.yml, so local lint matches CI

The Go version was stated in three places, which is exactly why all three drifted together. All three now point at a single source, so the prerequisites list cannot go stale.

Two notes on why the numbers earn so little in prose:

• Go corrects itself. go.mod has no toolchain directive, so under the default GOTOOLCHAIN=auto the toolchain downloads the required version automatically. A contributor on an older Go gets an upgrade, not a failure — it only hard-errors under GOTOOLCHAIN=local. (An earlier version of this description said such a contributor "cannot build"; that was too strong.)
• golangci-lint does not. Version skew here genuinely changes what you see: running CI's config under a newer golangci-lint (2.12.2) surfaces 23 goconst findings in cmd/publisher/commands/validate_test.go that CI's pinned 2.11.4 does not report. That is why the README now points at the pin rather than a number someone has to keep in sync.

make check does more than documented

Documented in three files as "lint, unit tests and integration tests". Actually check: dev-down lint validate test-all — it also runs schema validation and stops your running dev-compose environment. Worth knowing before running it mid-session. The Makefile's own ## help string already said this correctly; only the prose was stale.

Other verified corrections

• README project structure omitted cmd/registry/ — the server entry point and ko/goreleaser build target — and internal/importer/.
• tests/integration/README.md said the suite launches MongoDB. It launches PostgreSQL (postgres-integration-test), as the same file admits nine lines later. It also cited /v0/servers/{server_id}, a path removed a while ago; the test actually calls GET /v0/servers and GET /v0/servers/{serverName}/versions/latest (main.go:210,255).
• scripts/mirror_data/README.md pointed at internal/database/migrations/008_separate_official_metadata.sql, which does not exist — that migration is 009_, while 008_ is clean_invalid_data.sql. (Migration 008's own header still calls itself "007a", which is how the numbering slipped.) Also swapped two "edit line NN" pointers for identifier names, since both had already drifted (maxMigration is at :30 not :24; the connection string at :23 not :17), and corrected the fetch URL from /v0.1/servers to the /v0/servers the script actually requests.
• .env.example was missing MCP_REGISTRY_ENABLE_REGISTRY_VALIDATION, even though the config reads it (config.go:18) and README:50 tells contributors to set it while pointing at .env.example as the reference.
• docker-compose.yml set MCP_REGISTRY_ENVIRONMENT, which nothing reads — no ENVIRONMENT env tag in internal/config, and no other occurrence in the repo. Removed. (The Pulumi-side mcp-registry:environment key is a separate, real thing and is untouched.)
• Deleted CHANGES.md — 0 bytes, untouched since the squashed history base, zero references anywhere.

Verification

docker compose config validates. grep for Go 1.x / golangci-lint vX across all .md/.mdx now returns nothing outside complete.md (which #1522 deletes). Both ../../README.md#pre-requisites links resolve and the anchor matches the real #### Pre-requisites heading.

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