Add --version flag

[anmarchenko]

What

Adds ddtest --version, which prints the current ddtest version and exits successfully. Release and pre-release builds now embed the GitHub release name into the generated binaries, while local builds fall back to git describe or dev.

Why

CI jobs need a portable way to verify that they are running the expected ddtest binary before executing tests. This makes version checks consistent across runners and installation methods.

Fixes #69.

E2E testing

Build a temporary binary with an injected version and confirm the flag output:

go build -ldflags "-X github.com/DataDog/ddtest/internal/buildinfo.Version=v9.9.9" -o /private/tmp/ddtest-version-check main.go
/private/tmp/ddtest-version-check --version

Expected output:

v9.9.9

[anmarchenko]

E2E Test Report: SUCCESS

Tested by: Shepherd Agent (autonomous QA for Datadog Test Optimization)

Test Environment

• Method: Local testing from Shepherd
• Revision tested: 5caeb2ef03c05306be7d646eb33851e42734cdff
• Branch tested: anmarchenko/add-version-flag
• Playground: sidekiq with ddtest-plan

Results

Check	Status
PR binary builds with injected version ldflag	Passed
ddtest --version prints injected version and exits successfully	Passed
ddtest Go test suite	Passed
Shepherd crook builds PR branch as ddtest dependency	Passed
ddtest planning starts and completes in Sidekiq playground	Passed

Verification

• Built the PR checkout with:
  go build -ldflags "-X github.com/DataDog/ddtest/internal/buildinfo.Version=v9.9.9" -o /private/tmp/ddtest-version-check main.go
• Verified /private/tmp/ddtest-version-check --version returned v9.9.9.
• Ran go test ./... on the PR checkout; all packages passed, including internal/buildinfo and internal/cmd.
• Ran:
  ./bin/crook run sidekiq -c ddtest-plan --dep ddtest=anmarchenko/add-version-flag --debug
• Crook cloned and built the PR branch successfully, then ddtest plan discovered 36 test files and completed planning with 1 runner.

Issues Found

No PR-related issues found.

I also attempted a full sidekiq -c ddtest execution. The PR binary built and planning completed, but the Sidekiq test execution failed because Redis was not running at 127.0.0.1:6379. This appears to be local playground environment setup, not related to the --version change.

Datadog UI

Not applicable for this PR. The changed behavior is local CLI version output, and the planning-only validation intentionally does not emit test-cycle events.

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This E2E test was performed by Shepherd - autonomous QA agent for Datadog Test Optimization.