feat(ci): add opt-in conventional PR title gate

[trakhimenok]

Squash-merged runtime changes can pass CI but publish no release when their pull
request title is not a Conventional Commit subject. Add a migration-safe,
opt-in require_conventional_pr_title input to the shared Go workflow and run
the validation inside its existing required Lint job.

Failures show the invalid title, expected grammar, allowed types, valid examples,
release implications, and an exact gh pr edit command. The input is included
in exact-tree validation policy receipts. Existing callers remain unchanged
until they opt in.

Validation:

• go test ./...
• go vet ./...
• valid/malformed title regression matrix
• policy-digest opt-in regression
• git diff --check

Related WB client-side feature request: sneat-dev/wb#513

[trakhimenok]

Implementation review of exact head
a7aff7ccdd65f3b84b1134899de22561e91fef08: APPROVED.

No blocking findings.

• The new input defaults to false; existing callers do not change behavior
  until they opt in.
• Validation runs only for pull requests and inside the existing Lint job,
  before Go setup and expensive linting.
• The title enters Bash through an environment variable and is only displayed
  with printf %q; untrusted title contents are not evaluated as shell code or
  inserted into the workflow-command annotation.
• Failure output includes the grammar, allowed types, examples, release-type
  guidance, consequence, and exact gh pr edit remediation.
• The option is bound into exact-tree validation policy digests, preventing a
  receipt created under a different title policy from being reused.
• No permissions, release behavior, or push/tag behavior changed.

Validation passed: go test ./..., go vet ./..., focused valid/invalid-title
tests, policy-digest regression, seven stable remote checks, and diff checks.