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 needs the same durable-context workflow available in the Claude Code integration while preserving Pi-native package loading, explicit durable-write confirmation, and fail-open recall.

What changes are included in this PR?

• Add the native Pi package, setup/diagnostic support, CI validation, and real Pi CLI end-to-end smoke coverage.
• Align Pi's named tool surface with Claude Code's 18 agent-facing Server operations, including Handoff Reports and Artifact Candidate Review.
• Preserve scoped operation isolation, interactive confirmation and secret rejection for durable Candidate mutations, and unscoped Handoff Reports.
• Expose expected_revision and max_bytes controls, centralize tool registration, and document the complete surface in English and Chinese.

Are there any user-facing changes?

Users can install with powercontext setup pi and access native pc_* Memory, Handoff, Handoff Report, and Candidate Review tools in Pi. No breaking API or persisted-format changes.

How was this change tested?

• mise exec -- make pi-test
• mise exec -- make test (544 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; validation and review findings were incorporated before opening this draft PR.

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