[smart-log] feat: add identifier detection and prompts

[luqven]

Summary

Add identifier detection and interactive prompts for the smart-default log command.

Changes

New file: src/identifier.rs

• StackSummary - Display info for a stack (branch, PR count, numbers, title snippet)
• is_trunk_branch() - Check if branch is a trunk (main, master, develop, etc.)
• detect_trunk_branch() - Get default branch from git remote via git remote show origin
• TrunkAction enum - User choices when on trunk (enter identifier, select stack, cancel)
• prompt_trunk_action() - Interactive menu when user is on trunk branch
• prompt_select_stack() - Let user pick from a list of detected stacks
• prompt_identifier() - Manual identifier entry prompt

Modified: src/lib.rs

• Export new identifier module

Tests

12 new unit tests:

• test_is_trunk_branch_* - Various trunk detection scenarios
• test_stack_summary_* - Summary creation and display formatting

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