Verbosity Steering

# Verbosity steering — L0 through L4

These are the system-prompt fragments that implement Headroom's output-token
shaper. Append the selected level to the **tail** of the system prompt
(after any `cache_control` breakpoint, so the cached prefix stays warm).

The default level is **L2**. The five levels are cumulative: L2 = L1 + its
own rules; L3 = L2 + its own rules; etc.

> **Why the tail, not the head.** Prompt caching is a prefix match. Any byte
> change ahead of a `cache_control` breakpoint invalidates everything after
> it. Prepending steering text would bust the provider prefix cache and cost
> more than it saves. Appending after the last system block leaves the cached
> prefix byte-identical; only the small, byte-stable steering block is
> reprocessed.

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## L0 — off

(No text appended. Write naturally.)

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## L1 — no ceremony