feat: add optional workdir suffix for worktree disambiguation

[balgaly]

Summary

• Adds CFG_SHOW_WORKDIR, CFG_WORKDIR_STYLE, CFG_WORKDIR_MAX_LEN in both statusline.sh and statusline.ps1 (parity across shells).
• Line 1 renders as repo ▸ workdir in dim text when the workdir differs; the default worktree style is a no-op for single-worktree users.
• Four styles: full, relative (~-prefixed), basename, worktree (basename only when it differs from repo name — the user's case).
• Long paths are left-truncated with … so the worktree-specific tail always survives: …/code-koala-worktrees/p01-numbered-diffs.

Motivation: running many git worktrees off the same repo makes location_name (repo name) identical across them. The existing statusline gave no indication which worktree you were in.

Test plan

• New test suite test/test-workdir.sh — 11 tests covering all four styles, the disabled flag, single-worktree no-op, linked-worktree basename surfacing, and left-truncation preserving the tail.
• Existing suites still green: test-statusline-v2.sh (11), test-hooks.sh (7), test-brif-pane.sh (20). 49 tests total pass.
• PowerShell manual smoke test: verified full / relative / basename / worktree styles, home-dir ~ substitution works even when \$env:USERPROFILE and JSON-supplied cwd use different slash conventions.
• CI run on this PR.

Non-changes

• No change to other line-1 fields, colors, or layout.
• No worktree detection logic (as specified in the task brief — the worktree style is a simple basename-vs-repo-name comparison).
• No new JSON input fields.

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