refactor(version,git): derive the tag grammar from the canonical spec

[joshua-temple]

Problem

internal/version and internal/git each carried their own hand-written copy of the release-tag regex (semverRegex, baseVersionRegex, rcSuffixRegex, and git's versionTagRegex), policed only by a cross-package drift test. The git release classifier also decided "is this a published release" with a literal !strings.Contains(tag, "-rc."), which misclassifies any non-default prerelease token as a finished release.

Fix

Back both packages with the internal/taggrammar leaf spec introduced in #498, so the parser and the tag predicate can no longer diverge:

• version derives its parse and String() output from a taggrammar.Spec. Adds NewCalculatorWithGrammar, ParseWithGrammar, ParseBaseWithGrammar; the existing NewCalculator/Parse/ParseBase stay as default-spec wrappers so no call site changes. Default output is byte-identical.
• git delegates IsValidVersionTag to the spec (deletes the hand-copied regex) and adds IsValidVersionTagSpec. The release classifier now asks the spec "parses as a version with no prerelease segment" instead of matching a literal -rc., via a new GetLatestReleaseTagSpec. git imports only taggrammar (no version cycle).
• The cross-package drift test is replaced by a same-source assertion, since both sides now share one grammar.

No behavior change under the default grammar; this is the plumbing the configurable block wires into next.

Verification

• go build ./... clean
• go test ./... -race green (2471 tests)
• golangci-lint run ./internal/version/ ./internal/git/ clean
• git does not import version (confirmed via go list -deps)

Relates to #434.