Remove the `issue_intents` runtime feature gate

[alondahari]

Problem

Issue intent metadata for issue types, issue fields, and labels is currently gated on the issue_intents entry in GH_AW_RUNTIME_FEATURES. With the feature moving to public preview, workflows should no longer need that client-side environment-variable opt-in.

The server-side feature flag remains authoritative. Sending the intent-capable payload before that flag is fully enabled is safe: the metadata is ignored rather than producing an error.

Current implementation

The gate was introduced in #41092 and is implemented by hasIssueIntentsRuntimeFeature() in actions/setup/js/issue_intents.cjs. It currently controls:

• the intent guidance added to set_issue_type, set_issue_field, and add_labels tool descriptions in actions/setup/js/generate_safe_outputs_tools.cjs
• structured issue type updates and the GraphQL-Features: update_issue_suggestions header in actions/setup/js/set_issue_type.cjs
• issue field intent metadata and the feature header in actions/setup/js/set_issue_field.cjs
• structured label metadata sent by actions/setup/js/add_labels.cjs
• the GraphQL label intent path used by actions/setup/js/update_issue.cjs

Desired behavior

Remove the client-side issue_intents runtime feature check so intent-capable behavior is always used when these safe outputs run:

• Always include the intent guidance for set_issue_type, set_issue_field, and add_labels tools.
• For non-clear set_issue_type operations, always use the structured issue type input and feature header. Preserve the existing clear behavior.
• Always attach normalized rationale, confidence, and suggest metadata to issue field updates and send the feature header.
• Always preserve structured label metadata in add_labels requests.
• When update_issue includes labels, always use the existing GraphQL label intent path and feature header.
• Remove hasIssueIntentsRuntimeFeature() and its issue_intents constant/imports once no callers remain.
• Do not remove or otherwise change the generic GH_AW_RUNTIME_FEATURES infrastructure; other runtime features may still depend on it.

Compatibility requirements

• Existing string-only label payloads continue to work.
• Calls that omit rationale, confidence, and suggest continue to work without errors.
• Clearing an issue type continues to work.
• Existing sanitization, the 280-character rationale limit, confidence normalization, validation, allowed-list checks, and staged-mode behavior remain unchanged.
• Repositories or deployments where the server-side feature flag is not yet enabled do not fail; unsupported intent metadata is ignored.

Acceptance criteria

• GH_AW_RUNTIME_FEATURES no longer changes issue-intent tool descriptions or runtime mutation behavior.
• Tests no longer set GH_AW_RUNTIME_FEATURES=issue_intents to exercise issue intent behavior.
• Tests cover unconditional intent guidance and mutation paths with the environment variable absent.
• Tests cover string labels and payloads without optional intent metadata.
• No issue_intents runtime feature checks remain in the issue type, issue field, add-labels, update-issue, or tool-generation paths.