[smart-log] feat: make log and status identifier optional with branch inference

[luqven]

Summary

Enable gh-stack log and gh-stack status to work without an explicit identifier by inferring the stack from the current branch's PR chain.

New Usage

# Infer stack from current branch (NEW)
gh-stack log
gh-stack status

# Infer from specific branch (NEW)
gh-stack log --branch feat/my-feature
gh-stack status --branch feat/my-feature

# List all stacks, select interactively (NEW)
gh-stack log --all
gh-stack status --all

# CI mode - requires identifier or --branch (NEW)
gh-stack log --branch $GITHUB_HEAD_REF --ci
gh-stack status --branch $GITHUB_HEAD_REF --ci --json

# Existing behavior still works
gh-stack log 'STACK-ID'
gh-stack status 'STACK-ID'

Changes

Modified: src/main.rs

New CLI arguments for log and status commands:

• --branch, -b - Infer stack from specified branch instead of current
• --all, -a - List all stacks and select interactively
• --ci - Non-interactive mode for CI environments
• --trunk - Override trunk branch detection (default: auto-detect or "main")

Behavior changes:

• Running gh-stack log or gh-stack status without args infers from current branch
• On trunk branch: prompts to enter identifier or select stack
• No PR found: suggests gh pr create command with install hints
• CI mode requires explicit identifier or --branch flag (fails otherwise)
• JSON output mode handles errors gracefully with error field in response

Backwards Compatibility

The identifier argument is now optional but still fully supported. Existing scripts using gh-stack log 'STACK-ID' or gh-stack status 'STACK-ID' will continue to work unchanged.

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