docs: plan pnpm migration

[danshapiro]

Summary

• Add a comprehensive implementation plan for migrating Freshell from npm to pnpm.
• Cover workspace and lockfile design, command/lifecycle behavior, Electron runtime packaging, native Windows, CI/cloud/sandbox workflows, testing, documentation, rollout, and rollback.
• Link the plan from AGENTS.md while explicitly keeping the current npm workflow unchanged.

Investigation and verification

• Six read-only Luna xhigh investigations covered dependencies, packaging, CI/tests, documentation, command semantics, and deployment alternatives.
• Three independent Luna reviews checked the plan for packaging contracts, coordinator argv behavior, version enforcement, generated artifacts, and documentation completeness.
• The repository-supported npm baseline passed scripts/base-gate.sh test on the audited base: cloud Vitest, source-runtime smoke, Cargo workspace tests, and Electron tests.
• Current branch validation passed plan structure checks and git diff --check.

No pnpm migration was implemented in this PR; this PR contains the planning artifact only.