fix(diffr): faulty detection of claude on PATH

[sidkmenon]

Fixes the onboarding gaps from testing curl -fsSL https://install.dev.fast/diffr | sh end to end.

claude not on PATH

• config init treated Claude Code as installed when ~/.claude existed, then ran claude. If claude wasn't on PATH, setup died with could not run claude plugin marketplace list --json: No such file or directory.
• Now Claude Code is offered only when claude is on PATH. Without it, setup prints one line saying it skipped the Claude Code plugin and how to add it later, then finishes successfully.
• If the plugin was recorded as installed and claude is now gone, the record is left alone.
• --json lists Claude Code only when claude is on PATH, and the detected field is gone.

Re-running the install script never re-ran setup

• Before, a re-run only swapped the binary when diffr already existed. So installing Claude Code after diffr, or retrying a failed setup, did nothing.
• Now install.sh runs config init every time. Its defaults are the current state.
• diffr upgrade sets DIFFR_SKIP_SETUP=1, because it upgrades the plugins itself.

/diffr couldn't find diffr when Claude Code's PATH didn't include ~/.local/bin

• I reproduced this: it failed with ENOENT … install diffr with: curl …, and following that advice changed nothing.
• It happens when Claude Code is installed by Homebrew or npm and runs in a terminal opened before install.sh changed PATH, or when the PATH prompt was declined.
• For script installs, config init now sets the plugin's diffr option to the absolute path, via claude plugin configure diffr@devfast --values-stdin. Homebrew, Cargo and Whiteboard copies keep diffr, since their real paths change with each version and they're on PATH anyway.
• The plugin's could-not-run error now also says to run diffr config init.

Smaller fixes

• install.sh died on ${SHELL##*/} under set -u when SHELL was unset (dash, as in Docker or CI). It now uses ${SHELL:-}.
• For a shell it doesn't recognise, install.sh prints the exact line to add to PATH.
• A failed interactive setup now ends with "Run diffr config init to try again".

Testing

• cargo test. The CLI tests cover the missing-claude cases and check that the absolute path reaches plugin configure.
• Ran install.sh locally:
  • under dash with no SHELL and no terminal: it completes;
  • re-run in a pty: setup starts;
  • with DIFFR_SKIP_SETUP=1: setup doesn't start.
• Ran interactive config init with no claude on PATH. It skips, finishes, and exits 0.
• In an isolated CLAUDE_CONFIG_DIR, claude plugin configure --values-stdin sets the option for an installed plugin.

https://claude.ai/code/session_01HTHD79i4QdthzVCkDZ26gd