docs: decide desktop runtime packaging

[oratis]

Summary

• accept a Tauri-supervised target-specific Node 22 sidecar for the desktop runtime
• choose single-client JSONL stdio and keep multi-client daemon attachment out of v1
• add a reproducible no-PATH packaging/handshake probe
• record size, cold-start, signing, notarization, rollback, and DMG decision gates

Evidence

• current unsigned Tauri app: 6,733,824 bytes
• local target-thin runtime: 117,655,968 bytes
• thin/stripped runtime: 108,412,080 bytes
• isolated protocol v1 handshake with empty PATH: passed (~1.02s cold)
• signed/notarized release remains an explicit CI gate; this PR does not claim release credentials were available locally

Validation

• pnpm spike:desktop-sidecar
• pnpm docs:check
• pre-commit format/lint/typecheck/test (925 passed, 12 skipped)
• local pnpm --filter @deepcode/desktop tauri:build

Stack

Depends on #186.