Name cead's userspace and its three ways to fan out

[1zeroone0]

What this PR does

Two README clarifications of intent:

• The opening sentence names what kind of harness: an agent harness built as a microVM, a userspace whose user is a model. "Agent harness" stays the lead noun.
• Deployment's "unit of scale" bullet becomes a table of the three ways work fans out, told apart by who spawns and when: scale-out from a declaration (evals, operator, before the run), fork from a snapshot (RL, operator or trainer, mid-run), rlm (long or autonomous tasks, the model, whenever it decides).

What this PR does not do

• Frame cead as a Linux distribution.
• Change anything else in the README. "Forks like a process" stays pitch language.

Merge requirements

• Operator approves both changes.

🤖 Generated with Claude Code

[1zeroone0]

1. Built — Deployment's scale bullet replaced by a use / who / when / mechanism table.
2. Why — the three fan-outs (scale-out, machine fork, rlm) were being conflated; who spawns and when is what separates them.
3. Bloat — replaces a bullet rather than adding a section; the worktree analogy moved into the fork row.
4. Drift — the table is now the only statement of the use cases; the Horizon PR's comments point here rather than restating it.
5. Trust surface — prose only; nothing is testable yet.
6. How it breaks — the table promises scale-out and fork before either exists; both are Horizon items.
7. Not confident — whether "reinforcement learning" should say "operator or trainer"; whether the table belongs in Deployment or in "What cead is".
8. Verify yourself — the table rows against your intent.
9. Next steps — finished; ready for review.

[1zeroone0]

1. Built — the gloss moves into the lockup's centered block as a caption, after a <br>.
2. Why — GitHub joins soft-wrapped lines into one paragraph, so the gloss rendered as a dangler after the second sentence.
3. Bloat — no lines added outside the existing HTML block.
4. Drift — PR description updated.
5. Trust surface — not rendered locally; GitHub's HTML sanitizer allows <br> and <em> inside <p align>.
6. How it breaks — the lockup has a transparent ground with stone-colored letters, so on GitHub light mode both the logo and the dark caption sit on white; unchanged by this PR.
7. Not confident — whether you meant the caption under the logo or under the opening sentence.
8. Verify yourself — the rendered README top on the PR's Files tab.
9. Next steps — finished; ready for review.

[1zeroone0]

Caption commit moved to #3, which now owns the README's lockup block along with the assets. This PR is the framing text only.