[smart-log] feat: add stack discovery API

[luqven]

Summary

Add stack discovery API for finding PR stacks by walking base/head branch chains.

This enables gh-stack to find related PRs without requiring identifier patterns in titles - a key foundation for making the log command's identifier optional.

Changes

• New file: src/api/stack.rs - Stack discovery module with:

  • fetch_pr_by_head() - Find PR by head branch name
  • fetch_prs_by_base() - Find all PRs targeting a base branch
  • fetch_all_open_prs() - Get all open PRs in a repo
  • discover_stack() - Walk up/down to find full connected stack
  • discover_all_stacks() - Group all PRs into separate stacks

• Modified: src/api/mod.rs - Export new stack module

Algorithm

discover_stack works by:

1. Starting from a given PR
2. Walking UP: following base branches until hitting trunk
3. Walking DOWN: finding PRs whose base is in our visited set
4. Sorting results from bottom (closest to trunk) to top

Tests

9 new tests including mockito-based API tests:

• test_fetch_pr_by_head_found/not_found/rate_limited
• test_fetch_prs_by_base_multiple/empty
• test_fetch_all_open_prs
• test_sort_stack_linear/single/empty

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