Allow /resume to find sessions across all projects

Problem

When you start a conversation in one directory and later realize the work applies to a different project/repo, there's no way to resume that session from the new directory. Sessions are scoped to the cwd at launch, so /resume only shows sessions from the current project directory in ~/.claude/projects/.

This comes up frequently when:

• Work spans multiple repos
• You realize mid-conversation you're in the wrong directory
• You switch from a worktree back to the main repo (or vice versa)

Current workaround

Manually copy .jsonl files and session subdirectories between ~/.claude/projects/ directories, then /resume from the target directory:

cp ~/.claude/projects/<source-project-dir>/*.jsonl ~/.claude/projects/<target-project-dir>/
cp -r ~/.claude/projects/<source-project-dir>/<session-uuid> ~/.claude/projects/<target-project-dir>/

This works but is tedious and requires understanding the internal project directory naming scheme.

Proposal

Allow /resume to optionally search across all projects — either:

• By default when no local sessions match
• Via a flag like /resume --all to show sessions from all projects
• By accepting a session ID directly, regardless of which project it belongs to