fix: bootstrap pipx in Windows installer

[blisspixel]

Summary

• treat a missing pipx module as an expected Windows installer bootstrap condition
• add a regression contract and record the dated live-validation finding
• bump the patch release to 2.50.10 and synchronize package, plugin, docs, fixtures, and lock metadata

Validation

• live Windows PowerShell run bootstrapped pipx under Python 3.14 and installed the verified v2.50.9 GitHub Release wheel
• 29 focused installer, packaging, standards, and Agent Plugin tests passed
• Ruff lint and format checks passed
• docs consistency, file-size, C901, S, and dependency-audit gates passed
• Agent Plugin validation and build passed