CI hardening + release versioning + notifier/token drift fixes

[Topper (toppercodes)]

Summary

• upgrade repository Go target to 1.26.0 and align release workflow to go.mod
• use latest golangci/golangci-lint-action@v8 in CI lint job
• remove hardcoded provider user-agent version and inject providerVersion via ldflags
• derive Makefile VERSION from the exact git tag during releases (fallback to dev)
• fix GoReleaser ldflags package path for provider version injection
• make make generate robust in branch/worktree checkouts by pinning tfplugindocs provider naming in go:generate
• treat wrapped/typed 404 responses consistently so missing resources are recreated instead of hard-failing
• preserve PagerDuty notifier credentials in state when API responses omit/redact them to avoid post-apply inconsistent-state errors
• add unit tests for provider user-agent version formatting, not-found detection, and notifier PagerDuty state merge behavior

Validation

• make build
• make generate
• go test ./axiom -run TestProviderUserAgent -count=1
• go test ./axiom -run TestIsNotFoundError -count=1
• go test ./axiom -run TestMergeNotifierState -count=1
• $(go env GOPATH)/bin/golangci-lint run --timeout=5m --output.text.path=stdout

Notes

• go test ./... currently executes acceptance-style tests in axiom/provider_test.go and requires token/env setup; this behavior is pre-existing and unchanged by this PR.

Fixes #85
Fixes #54
Fixes #78