fix: make harmonic backend optional

[warren618]

Summary

Fix Windows/Python baseline installs by moving the optional pyharmonics harmonic-pattern backend out of the core dependency set and into a dedicated harmonic extra.

Root Cause

vibe-trading-ai depended on pyharmonics>=1.5.0 unconditionally. pyharmonics depends on ta, which can be unavailable in some Windows/Python resolver environments, so a non-core harmonic-pattern backend could block pip install vibe-trading-ai entirely.

Changes

• Remove pyharmonics from core pyproject.toml dependencies and agent/requirements.txt.
• Add pyharmonics>=1.5.0 as vibe-trading-ai[harmonic].
• Update the harmonic skill install instructions to use the extra and document the bundled fallback detector.
• Add a packaging regression test that keeps pyharmonics out of baseline installs while preserving the optional extra.

Fixes #249.

Validation

• python -m pytest agent/tests/test_packaging_dependencies.py -q
• python -m compileall -q agent/tests/test_packaging_dependencies.py
• python -m pip install --dry-run --ignore-installed --no-deps .
• git diff --check