refactor(safe-outputs): reduce complexity of execute_impl in create_github_issue.rs

[github-actions]

What

CreateGithubIssueResult::execute_impl in src/safe_outputs/create_github_issue.rs was flagged by Clippy's too_many_lines lint at 168/100 lines. This PR extracts three logically distinct blocks into dedicated helper functions, with no behavioural change:

• validate_agent_labels(labels, config) -> Result<(), String> — the allowed-labels allowlist check (default-deny semantics, wildcard support, disallowed-label error formatting).
• build_final_title(title, config) -> Result<String, String> — applies title-prefix and enforces MAX_FINAL_TITLE_LEN.
• CreateGithubIssueResult::handle_issue_response(ctx, target_repo, response) -> anyhow::Result<ExecutionResult> — interprets the GitHub API response (success/failure, issue number/URL extraction, temporary_id registration).

execute_impl itself is now a straight-line sequence of guard checks followed by delegation to these helpers, which are independently readable and (in principle) independently testable.

Before / after

• Before: clippy::too_many_lines warning, 168/100 lines, on execute_impl.
• After: no too_many_lines (or any other) Clippy warning for this file.

Verification

• cargo check --all-targets — clean.
• cargo clippy --all-targets --all-features — clean, no warnings anywhere in the crate.
• cargo test --bin ado-aw — full suite: 3360 passed, 0 failed, 1 ignored.
• cargo test --bin ado-aw create_github_issue — all 46 module tests pass unchanged.
• rustfmt applied to the touched file only; no unrelated formatting changes.

No public API signatures changed; no observable behaviour changed.

Warning

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[jamesadevine]

Closed as partially superseded by aggregate PR #2189, which retains the isolated create_github_issue complexity refactor. Unrelated or overlapping cumulative changes were intentionally omitted.