feat(pi): add native Pi Coding Agent support

[pi-dal]

Which issue or RFC does this PR close?

Closes #1258

Rationale for this change

Pi Coding Agent currently has no PowerContext integration. This adds the smallest native workflow that restores durable project context and transfers work between Pi sessions. It is deliberately not a field-for-field or operation-for-operation clone of the Claude Code integration.

What changes are included in this PR?

• Add the native Pi package, setup/diagnostic support, CI validation, and a real Pi CLI end-to-end smoke test.
• Restore bounded context before normal prompts and capture eligible prompt evidence without syncing Pi transcripts.
• Expose named pc_* Memory and Handoff tools, with interactive confirmation and secret rejection for durable writes.
• Require confirmation for explicit /pc flush operations.
• Share credential-safe GitHub source validation between Pi and DSH setup, rejecting credential-bearing URLs and redacting clone failures.
• Centralize tool registration and document installation, configuration, and the supported workflow in English and Chinese.

Are there any user-facing changes?

Users can install with powercontext setup pi and access native pc_* Memory/Handoff tools and /pc diagnostics in Pi. No breaking API or persisted-format changes.

How was this change tested?

• mise exec -- make pi-test
• mise exec -- make test (547 passed, 1 skipped)
• mise exec -- make contract-test
• mise exec -- make docs-test
• mise exec -- make check

AI usage statement

Implemented with Codex (GPT-5) under user direction.

[CLAassistant]

All committers have signed the CLA.