fix(coding-agent): use Windows venv interpreter

[alloutflo]

Summary

Fix Windows kernel bootstrap by resolving the virtual-environment interpreter from the platform-specific layout.

Root cause

The bootstrap path always used <venv>/bin/python. uv creates <venv>/Scripts/python.exe on Windows, so both the initial package installation and later readiness checks addressed a path that does not exist.

Changes

• Add one platform-aware kernel-venv interpreter path helper.
• Use it when bootstrapping the venv and when checking or reusing an existing venv.
• Add focused coverage for Windows and POSIX layouts.
• Document the fix in the coding-agent changelog.

Verification

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/kernel-bootstrap.test.ts — 22 tests passed.
• npm run check — formatting, linting, type checking, installer rendering, and browser smoke checks passed.

Risk

Low. POSIX keeps the existing bin/python path. Windows now uses the standard Scripts/python.exe path. The platform selection is covered without requiring a Windows host, but this PR does not claim a full Windows end-to-end install run.

Fixes #1194

Note

Fix kernel bootstrap to use Windows venv interpreter path

The kernel bootstrap always used the POSIX bin/python path, causing failures on Windows where the interpreter lives at Scripts/python.exe.

• Introduces getKernelVenvPythonPath in bootstrap.ts to return the correct interpreter path based on platform (Scripts/python.exe on Windows, bin/python on POSIX).
• Updates bootstrapVenv and ensureKernelPythonUncached to use this utility for dependency installation, readiness checks, and the final returned interpreter path.
• Adds a unit test in kernel-bootstrap.test.ts covering both platform layouts.

^{Macroscope summarized 8c3fd03.}