[smart-log] feat: add gh CLI helpers

[luqven]

Summary

Add gh CLI detection and installation helpers to help users create PRs when none exist for a branch.

Changes

New file: src/gh_cli.rs

• is_gh_installed() - Check if gh CLI is available
• is_gh_authenticated() - Check if gh is logged in
• install_instructions() - Platform-specific install commands (macOS/Linux/Windows)
• suggest_create_pr() - Print gh pr create command with installation hints
• prompt_create_pr() - Interactive PR creation prompt

Modified: src/lib.rs

• Export new gh_cli module

Behavior

• Gracefully handles non-TTY environments (CI)
• Shows installation hints when gh is not installed
• Shows auth hints when gh is installed but not authenticated
• Interactive prompt when running in a terminal

Tests

3 unit tests for basic functionality.

Stacked PR Chain: smart-log

PR	Title	Merges Into
#35	feat: add stack discovery API	-
#36	feat: add rate limit retry logic	#35
#37	feat: add identifier detection and prompts	#36
#38	👉feat: add gh CLI helpers	#37
#39	feat: make log and status identifier optional with branch inference	#38
#40	docs: update log and status documentation for smart defaults	#39
#41	perf: optimize stack discovery with batch fetch	#40
#42	perf: parallelize status check fetches	#41

[luqven]

Landed via #42