refactor: reduce complexity of execute_impl in create_pull_request.rs

[github-actions]

What was complex

execute_impl in src/safe_outputs/create_pull_request.rs had grown to 883 lines — a massive single method handling every phase of PR creation inline:

• Patch validation (file existence, size, SHA-256 check, path security, protected files)
• No-changes handling with IfNoChanges dispatch
• Agent label validation against allowlist + merge with operator labels
• ADO branch push with TOCTOU collision retry (using duplicated serde_json::json! block)
• Branch renaming logic inlined in two separate places

The function was readable locally but too long to hold in working memory, and the duplicated branch-suffix logic was a DRY violation.

What was changed

Six focused helpers extracted from execute_impl:

Helper	Lines extracted	Responsibility
handle_no_changes	~54	Builds symlink suffix, dispatches on IfNoChanges variant
validate_and_build_labels	~44	Validates agent labels against allowlist, merges with operator labels
push_new_branch	~70	ADO REST push + single TOCTOU-collision retry
build_push_payload	~15	Pure: builds the ADO push JSON body
random_branch_suffix	~5	Encapsulates rand::rng().random()
rename_branch_suffix	~7	Replaces duplicated rfind('-') / format! pattern

PushBranchParams struct introduced to satisfy Clippy's too_many_arguments lint on push_new_branch.

Before / After

	Lines
Before	883
After (execute_impl only)	718
Total extracted	165 (into 6 helpers)

Verification

• cargo build — clean
• cargo test — all tests pass
• cargo clippy --all-targets --all-features — 1 pre-existing warning in common.rs (unrelated), no new warnings

Warning

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