Workspace Anchor Cache Contract

The session workspace_anchor is transient turn context, not durable system prompt content.

The contract is:

• SessionState::system_prompt must stay free of {{workspace_*}} and {{project_*}} template tokens.
• active workspace state is rendered through the canonical workspace_anchor reminder provider instead
• downstream callers that still assemble pipeline_input or system prompt text by hand must keep workspace/project fields out of the persisted session prompt surface

The canonical reminder body is:

<workspace-anchor>
primary: /Users/me/projects/X
additional_roots:
  - /Users/me/projects/Y (mount_mode: read_only, mounted_at: 2026-05-23T10:15:00Z)
</workspace-anchor>

That reminder is injected on SessionStart and refreshed after each turn via the OnBudgetThreshold provider pass. Re-anchoring therefore updates the next turn's reminder payload without rewriting the stored session prompt.

In debug builds, Harn asserts this contract when it records a session system prompt. If the prompt text contains {{workspace_*}} or {{project_*}}, the runtime panics with HARN-CACHE-001 so the offending caller can move that data into reminder space instead.