feat(draftassist): inject DraftAssistHost; stop spawning from NewHTTPHandler

[AlexsanderHamir]

Summary

• Resolve hamix-draft-agent via HAMIX_DRAFT_AGENT_BIN, executable sibling, then PATH.
• NewHTTPHandler takes an injected DraftAssistHost; the smoke test supplies Fake so CI never spawns the sidecar.
• Production selection still degrades to Fake when the sidecar or CURSOR_API_KEY is missing (P2 removes that fallback).

Test plan

• .\scripts\check.ps1 -GoOnly
• Merge via gh stack merge from feat/draft-assist-inject (full stack: feat(draftassist): inject DraftAssistHost; stop spawning from NewHTTPHandler #201 → feat(draftassist): fail taskapi boot unless sidecar is healthy #202 → docs(draftassist): require CURSOR_API_KEY; honest compose banner when taskapi is down #203)

Notes

• Fake runner remains a test seam. P2 makes production fail-boot; do not merge P2 onto main without P3.