docs(readme): install trail from main until @trailhq/trail is on npm

[rushabhnagda11]

Found while rehearsing the new onboarding for the 2026-10-19 metrics review.

What

• npm install -g @trailhq/trail, the first line of the Trail quick start, returns 404: the package isn't published.
• The fallback ("Trying this branch…") cloned feat/trail-cloud from trailhq/Graft.git. That branch lacks feat: trail init learns from past Claude sessions, and signing in waits #585 (src/notes/cold-start.ts doesn't exist there), so a new user following the README gets a trail init that never learns from past sessions.

This PR points the fallback at main of trailhq/Trail-Code and says plainly that the package isn't on npm yet. The npm line stays as the intended path once it's published.

Test

Ran the new three lines verbatim from a clean clone into an empty npm prefix: exit 0 in about 60s, trail --version → 0.21.1, and cold-start.ts is present.

Not in this PR

Publishing @trailhq/trail (npm publish build/trail --access public) is the real fix. Once it's out, this paragraph can go.

🤖 Generated with Claude Code

[trailhq-graft]

🌱 graft blast radius

Nothing outside this diff depends on it. 0 areas changed; no indexed dependents at depth 2.

⚠️ 1 changed file not in the graph (README.md) — no parser claims the extension, or the index predates the file.

_{graft blast · origin/main...HEAD · depth 2 · 1 changed file}

Open the interactive graph → — click an area to see its dependent symbols at file:line.

[rushabhnagda11]

Superseded by #595: trail now publishes as @trailhq/trail-code and the README quick start installs it from npm. Closing.