fix(types): make context builder output attachable

[LukasParke]

What

• add structural ContextInput (readonly ContextLayer[] or any object with readonly layers)
• accept it at spawn and withContext type/runtime seams
• make context([...]) output directly assignable without losing its inferred shape
• update example helper seams, specs, docs, and agent-builder references

Why

ContextConfig preserves exact layer types and is therefore invariant. The output of the public context() builder was not assignable to the context option on the builders it is meant to feed. A shallow structural input type preserves inference while giving attachment seams the minimum shape they need.

This semantically ports fork/port/openrouter-fixes commit efc2b689 onto post-#68 withContext/current names without restoring deleted bundled patterns.

Test plan

• compile/runtime ContextInput tests — 13 pass
• types/context/core typechecks
• examples typecheck
• root lint
• export-tag check
• sentrux check .
• clean-context Pi review; fixed docs, delegate helper seam, and object-branch runtime coverage

Compatibility

Additive type widening only. Existing arrays and ContextConfig values remain valid.

[vercel]

@LukasParke is attempting to deploy a commit to the Matt Apperson's projects Team on Vercel.

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[LukasParke]

Agent: CI reached the full core suite and failed only the unrelated timing-sensitive executeRunCode retry backoff test (fixed backoff or linear backoff depending on the run). The affected package tests/typechecks and local full suite passed. The separate compat failure is the repository workflow receiving an empty OPENROUTER_API_KEY, and Vercel requires upstream authorization. I cannot rerun upstream Actions without repository admin rights.

[LukasParke]

Agent: Root-caused the repeated core CI failure to retry tests proxying global setTimeout and capturing sibling-test timers. Pushed a deterministic fix that tests the pure retry-delay function without mutating globals. DCO and Structural gate pass; CI rerun is in progress.