Prompting improvements

[MathisWellmann]

• Tighten up prompting and prefix nudges with nudge:
• Increase maximum number of doc tool calls from 8 to 50
• Extract out Error::to_nudge from Runtime::evolve_lane
• remove first_line_of which just swallowed the error message too much.
• include tool call history if max tool call error is triggered by rig, to make recovery easier.
• Include EvolveFailure::UnimplementedFunction and provide backpressure for empty implementations.

[github-actions]

Summary

Adds an UnimplementedFunction error (stub/echo body rejection in validation.rs), centralizes nudging in Error::to_nudge, recovers partial transcripts from rig's aborted-run errors into the retry history, and raises the doc-tool turn budget to 50. Verified against the vendored rig/syn sources, the core logic is sound; remaining defects are leftover debug output and overpromising docs.

Issues

• Major — symbiont/src/validation.rs:1251,1340,1459,1476: The diff adds eprintln!("DBG code: ...") into shared helpers assert_rejects_unsafe/assert_rejects_forbidden and two tests, so every unsafe/forbidden validation test prints debug noise to stderr. Remove these four leftover debug statements before merge.
• Major — symbiont/src/error.rs:167-177, symbiont/src/runtime.rs:406-410: The new doc comments claim transcript recovery for PromptCancelled/UnknownToolCall lets a retry extend the aborted run, but Error::to_nudge (symbiont/src/error.rs:153) treats both variants as terminal, so the lane ends without a retry and their appended messages are never used. Correct the docs (or nudge those variants) to match behavior.

[github-actions]

Summary

The PR adds a new UnimplementedFunction rejection for stub/echo bodies, moves nudge construction into Error::nudge, and recovers aborted tool-loop transcripts into the retry history. Two major defects: the empty-body check contradicts the new partial-evolution contract, and the repeat-reset prompt now re-quotes the rejected code.

Issues

• symbiont/src/validation.rs:177 (major) — check_implementation_bodies rejects an empty body unconditionally, even when the declared default body is itself empty (fn f() {} is legal in evolvable!). A host with an empty default can then never land a partial evolution that keeps it empty, contradicting the function's own doc ("A verbatim echo … only rejects when no declared function changed") and the new system_prompt.rs line "the others may keep their default bodies"; the doc's "panics or fails to compile" rationale is also false for {}. Fix: only reject an empty body when the stored default for that function is non-empty, treating empty==empty as an echo.
• symbiont/src/runtime.rs:1418 (major) — The repeat-reset prompt now embeds the full error (e) where first_line_of(&e) was deliberately used, because for CompilationFailed the display includes rustc output that quotes the rejected source — exactly the echo source the reset is meant to stop the agent from repeating; the backpressure_repeat.rs assertion that the reset prompt must not quote the rejected code was deleted along with it. Fix: keep a code-free summary in the prompt (restore first_line_of or equivalent) and use the full error only in the trace event.