[status-command] Add status module with core logic and rendering

[luqven]

Summary

Add status.rs with the core status functionality:

• StatusBit enum (Passed/Failed/Pending/NotApplicable)
• PrStatus struct with CI/approval/mergeable/stack_clear bits
• build_status_entries() - fetch and compute status for stack
• render_status() - format output with status bits (unicode/ASCII)
• render_status_json() - JSON output format
• Legend detection via ~/.gh-stack-legend-seen (first-run only)
• Title truncation to 50 chars

Output Format

◉ feature-3 (current) #125 - Add final feature...
│ [✓ ✗ ✓ ✗]  2 hours ago
│
│ - abc1234 Add widget component
│
◯ main

Status: [CI | Approved | Mergeable | Stack]

Tests

38 tests including:

• 29 unit tests for status bit logic, formatting, stack computation
• 9 snapshot tests for output rendering (ASCII, Unicode, JSON)

Dependencies

• Adds dirs crate for home directory detection
• Adds Serialize derive to CommitInfo for JSON output

---

Part 3 of 5 in the status-command stack

Stacked PR Chain: status-command

PR	Title	Merges Into
#30	Add head_sha/html_url accessors to PullRequest	-
#31	Add GitHub Checks API module	#30
#32	👉Add status module with core logic and rendering	#31
#33	Add status subcommand and log --status flag	#32
#34	Add status command documentation	#33

[luqven]

Landed via #34