feat(shell): add canonical shell and PowerShell backend

[huangzesen]

Summary

• make shell the single canonical public tool/capability while accepting legacy bash only as a one-way input/config compatibility alias
• expose the selected native shell dialect in the tool description (posix on macOS/Linux, powershell on Windows)
• add a native PowerShell 7 adapter for the existing sync/async/timeout/cancel/stdout/stderr/exit-code contract
• add Windows process ownership with Job Objects and a native cross-process state lock
• add a separate pull-request Windows workflow with real pwsh integration tests

Scope / non-goals

This is PR1 only. It does not add POSIX-to-PowerShell translation, ExecSpec, ConPTY/TUI/bootstrap/distribution work, or a generic Windows service layer. The retained internal Python package and durable bash.reminder / bash.completion notification namespaces stay compatibility-stable.

Validation

• 227 passed, 5 skipped — focused capability, async, dialect/process, daemon preset, layer, skill, and PR1 tests (the five real Windows tests skip on macOS)
• 169 passed — anatomy drift + tool glossary governance in isolation
• python -m compileall -q ... passed
• workflow YAML parse passed
• git diff --check passed
• exact live base before commit: origin/main = fda6d40c467dea9472ee1bdde36d93278be81058

The native Windows/PowerShell proof is intentionally delegated to the new windows-latest PR job; the local host has neither Windows nor pwsh, so no mock result is presented as native evidence.

Existing baseline-only observations

A broader docs-governance run also had two unrelated failures in unchanged inputs: missing frontmatter in RELEASING.md and a stale Telegram notification-body hash expectation. A mixed test batch exposed pre-existing unclosed-file warnings from daemon test fixtures inside a glossary warning-count assertion; the glossary/anatomy family passed cleanly when run in isolation (169 passed).

Review order

The PR is ready for native Windows CI and independent exact-head review. Merge is intentionally not authorized by this PR.