feat(hosts): add native Pi and WorkBuddy plugin runtimes

[NickyLam]

Summary

Adds native Pi and WorkBuddy plugin runtimes with a shared three-lane host contract:

• native Pi /better-harness extension;
• native WorkBuddy Marketplace Shell with one director and three lane-specific members;
• host doctor/prepare/verify flow, lane isolation and input hashes;
• packaging, discovery, documentation, and regression coverage while preserving canonical evidence/report ownership.

Why

The existing integrations exposed partial capabilities but lacked a host-native, independently isolated three-lane review flow and complete installation/discovery contracts. This change makes both integrations locally installable and marketplace-ready.

Traceability and Scope

• Specs: docs/specs/2026-08-06-better-harness-host-runtime-contract.md, docs/specs/2026-08-06-pi-native-runtime-completion.md, docs/specs/2026-08-06-workbuddy-expert-plugin.md
• Acceptance coverage: doctor/prepare/verify, exactly three isolated lanes, input hashes, current-session exclusion, quick/normal failure semantics, Pi command discovery, WorkBuddy expert-team discovery, and artifact/package validation.
• Non-goals: remote npm/Marketplace publication, version/changelog/roadmap changes, and claims of real model-driven host E2E where host executables are unavailable.

Test and Review Evidence

Check	Result
npm test	1060/1060 passed
npm ci --ignore-scripts	passed
npm run pack:verify	npm package 384 entries; runtime bundle 394 entries
npm run workbuddy:verify	4 agents; 3 members
npm run workbuddy:pack	380-entry archive generated
doc-link graph	34 files; 50 links validated
git diff --check	passed
isolated Pi 0.83.0 discovery	passed; one canonical /better-harness command

A local codebuddy executable was unavailable, so WorkBuddy CLI/Team E2E could not be run. Real model E2E, Node 22/24 matrix, and the official WorkBuddy validator remain explicit Draft PR boundaries.

Risk and Recovery

• Cross-platform path/process behavior is covered by contract tests; Windows/Linux host smoke was not available here.
• Revert commit: f74b3c79cde70ccc4bdbb0ccf8a8b9084325d785.
• Final reports remain findings.json, report.md, and report.html.

AI Involvement

• Level: Assisted.
• Human review covered repository tests, packaging/discovery, doc links, diff hygiene, and explicit host-dependent boundaries.

Checklist

• Focused diff; unrelated local worktree changes were excluded.
• Tests and documentation match delivered behavior.
• Markdown links and package/runtime contents were validated.
• Windows, macOS, and Linux compatibility was considered.
• Contribution is under the repository MIT License.
• CHANGELOG.md update (intentionally held for maintainer review per the implementation plan).