fix(claude): correct path tilde and add PR number segment to statusline

[fnayou]

Summary

Two fixes to the Claude Code statusline (stow/common/claude/.claude/statusline-command.sh):

1. Path tilde — the home shortening rendered a literal backslash (\~/works/dotfiles) because the bash parameter-expansion replacement string kept the \ verbatim. Use a plain ~ → ~/works/dotfiles.

2. PR number segment — new GitHub segment showing the open PR number for the current branch (nf-fa-github icon + #N, mauve #c6a0f6):

   • gh pr view is a ~0.5s network call — too slow per render. Cached per repo+branch in ~/.cache/claude-statusline/ with a 120s TTL and refreshed in the background, so the statusline never blocks. The number appears one render after a branch first gains a PR.
   • Icon stored as an octal escape (\357\202\233) and printf'd, matching the OS-icon style.
   • Shows nothing when the branch has no open PR (closed/merged/none → empty cache).

Test

• bash -n clean.
• Path renders ~/works/dotfiles (no backslash).
• On a worktree checked out to a branch with an open PR, the segment renders #34; branches without a PR render nothing.

Out of scope (noted, not changed)

The path-truncation code (lines ~37–41) uses negative array indices (${parts[-3]}), which need bash ≥4.3 — a pre-existing limitation, not introduced here.