Add github_pr item with OSC 8 clickable link and nerd font icon

[JefStat]

Summary

• Adds a new github_pr prompt item that displays the GitHub PR number for the current branch
• PR number is rendered as an OSC 8 clickable hyperlink to the PR URL (e.g., #1234 links to the PR page)
• Uses nerd font icon U+F408 (git-pull-request) via tide_github_pr_icon universal variable
• Fetches both number and url in a single gh pr view call with jq: "\(.number)|\(.url)"
• Uses command substitution instead of piped read to reliably capture both fields
• 5-minute file-based cache keyed by git toplevel + branch name, with platform-aware stat for TTL (Darwin/Linux)
• Cache format: number|url (e.g., 1234|https://github.com/owner/repo/pull/1234)
• Validates cache content before rendering (count check, non-empty parts, string split -m 1)
• Includes theme colors for all 6 config variants (rainbow, classic, lean + 16color) and icon in icons.fish
• Includes littlecheck tests covering: gh not installed, not in git repo, no PR, with PR, cached result, cached negative result

Test plan

• fish --no-execute passes on all new/modified files
• fish_indent --check passes on all new/modified files
• littlecheck tests pass for _tide_item_github_pr.test.fish
• OSC 8 byte sequence verified correct via od (ESC]8;;URL ESC\ #NUM ESC]8;; ESC)
• Verify clickable link renders in a terminal with OSC 8 support (iTerm2, Kitty, etc.)
• Verify cache TTL expiry works (wait 5 minutes or modify mtime)
• Verify no output when gh is not installed or no PR exists

🤖 Generated with Claude Code