Makefile release target and README refresh for the one-way dependency

[TopherGopher]

Part of the easymongo refactor (see PLAN.md on the refactor branch).

Working context (read this first)

• Repository github.com/tophergopher/easymongo. Start from branch claude/mongotest-easymongo-refactor-wu2vhm; create a branch issue-<number>-<short-slug> from it and open a pull request back into it. PLAN.md on that branch is the source of truth; read it before writing code.
• Go project, single module, all packages at the repo root except the new easymongotest/ subpackage. The final toolchain target is Go 1.27 (set by Toolchain: go 1.27, latest dependencies, tidy, remove replace directives #10); any Go 1.24 or newer works until then. Before pushing: gofmt -l . prints nothing, go vet ./..., go test -race ./....
• The existing test suite (*_test.go at the root, package easymongo_test, every test function calling setup(t) from common_test.go) is the regression net for this refactor. It must pass after your change. It needs a Docker daemon reachable through the normal Docker environment and the mongo:8 image.
• Order matters. The driver package cannot compile half-migrated (the *mongo.Client type flows from Connection to Database to Collection), so the four driver issues Driver v2: migrate connect.go (mongo.Connect, Disconnect, BSONOptions, write concern) #2, Driver v2: options struct literals become builders; drop MaxTime; ArrayFilters and Delete option changes #3, Driver v2: primitive package merged into bson; replace x/bsonx in index.go #4 and Driver v2: Distinct returns a DistinctResult that must be decoded #5 land together in one pull request, worked in that order. Before them, New easymongotest subpackage wrapping mongotest/v2 #7 and Switch the easymongo test suite to easymongotest #8 must land so the test suite no longer depends on the old github.com/tophergopher/mongotest v0.1.0 module (which itself imports easymongo and breaks the moment easymongo's types change). Until mongotest's v2 module is tagged, use a local replace github.com/tophergopher/mongotest/v2 => ../mongotest/v2 in go.mod and say so in the pull request; Toolchain: go 1.27, latest dependencies, tidy, remove replace directives #10 removes it.
• Method is test-driven: write the tests listed under "Tests first", confirm they fail for the expected reason, then implement. Never delete, skip or weaken an existing test to get green; if a test must change because the public API changed (for example primitive.ObjectID to bson.ObjectID), change only the types.
• Driver v2 reference: https://pkg.go.dev/go.mongodb.org/mongo-driver/v2 and the migration guide https://github.com/mongodb/mongo-go-driver/blob/master/docs/migration-2.0.md. Signatures quoted in these issues were checked against v2.9.0.
• Do not add third-party dependencies unless this issue lists them.
• Tracking issue for the whole refactor: Tracking: easymongo refactor (mongo-driver v2, easymongotest helpers, toolchain) #13. The mongotest side is tracked in Tracking: mongotest refactor (stdlib Docker client, driver v1/v2 modules, lighter dependencies) mongotest#48.
• Commit messages: imperative summary line, blank line, body explaining why. Mention this issue number in the pull request description.

Scope

• Makefile: the release target currently edits ../mongotest/go.mod, commits and tags the sibling repo in lock-step. mongotest no longer depends on easymongo, so remove that half. New behaviour: given TAG=vX.Y.Z, optionally MONGOTEST=vA.B.C to bump the github.com/tophergopher/mongotest/v2 requirement first, then go mod tidy, commit, tag, push tags. Use a portable sed invocation (the current sed -i '' is macOS-only).
• README: replace every mongotest.NewTestConnection example with easymongotest.Run(t) / easymongotest.NewTestConnection, update the import path shown to github.com/tophergopher/easymongo/easymongotest, mention bson.ObjectID instead of primitive.ObjectID in the struct examples, add the TLS connection snippet, and note the driver v2 requirement (Go 1.27).
• mongo.code-workspace is gitignored by pattern but committed; either remove it from the repo or drop the ignore rule. Recommend removing the file from git.
• Add a CHANGELOG.md entry for the minor release describing the bson.ObjectID API change and the new subpackage.

Implementation details

Portable in-place sed: use sed -i.bak ... && rm go.mod.bak (works on GNU and BSD sed). Release target sketch:

release:
ifndef TAG
	$(error TAG is required, e.g. make release TAG=v0.2.0)
endif
ifdef MONGOTEST
	go get github.com/tophergopher/mongotest/v2@$(MONGOTEST) && go mod tidy
	git add go.mod go.sum && git commit -m "Bump mongotest/v2 to $(MONGOTEST)"
endif
	git tag $(TAG) && git push origin master --tags

With DRY_RUN=1, prefix every command with echo.

Acceptance

• make release TAG=v0.2.0 dry-runs correctly on Linux and macOS (document a DRY_RUN=1 flag that echoes instead of executing).
• README examples compile when pasted into a test file.

Depends on: #11.