fix(plugin): fall back to pip/uv-installed skillopt-sleep in run-sleep.sh

[ichoosetoaccept]

Summary

The Claude Code and Codex plugin shells' runner (run-sleep.sh) only resolved the engine by searching for a skillopt_sleep/ source directory on disk. After v0.2.0 shipped the engine on PyPI (skillopt-sleep CLI), the plugins still errored for users who installed via pip/uv without cloning the repo — even though the error message said "pip install skillopt" would work.

This adds two fallbacks before the error:

1. skillopt-sleep CLI on PATH (covers uv tool install, pipx, pip install) — checked first because uv tool install / pipx isolate the package from the system Python's import path, so the import check would fail even though the CLI is available.
2. python -m skillopt_sleep when importable (covers pip install into the active Python)

The existing source-checkout resolution path is unchanged — repo-clone users keep working as before. The fix lands in both plugins/run-sleep.sh (shared, used by Codex) and plugins/claude-code/scripts/run-sleep.sh (bundled copy, byte-identical, used by the Claude Code marketplace install which fetches only the plugins/claude-code/ subtree).

After this fix, the install path is:

• uv tool install skillopt (or pip install skillopt) — self-updating engine
• /plugin marketplace add + /plugin install — self-updating plugin shell
• No repo clone, no SKILLOPT_SLEEP_REPO, no manual git pull.

Closes #106.

Test plan

• tests/test_run_sleep_fallback.py — 6 new tests covering source-checkout path, CLI fallback (both shared and bundled copies), arg passthrough, no-engine error message, and bundled/shared sync check
• shellcheck passes on both modified scripts
• Source-checkout path verified: bash plugins/run-sleep.sh --help from repo root works
• CLI fallback verified: uv tool install skillopt, then run from /tmp with no source dir — engine runs via skillopt-sleep on PATH
• No-engine error verified: run from temp dir with empty PATH — new error message mentions all three install options
• Full test suite passes (8 pre-existing import errors from missing pytest/openai optional deps, unrelated to this change)

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