feat: add agent loop resilience (§9.8, §9.9, §9.10, §12.5)

[sethjuarez]

Summary

Adds three new resilience features to the agent loop spec and implements them across all four runtimes (Rust, Python, TypeScript, C#), based on consumer integration feedback from a Tauri desktop application.

Spec Changes (spec/spec.md)

Section	Feature	Level	Description
§9.8	Resilient Argument Parsing	SHOULD	4-strategy JSON fallback: direct → strip markdown fences → extract {...} block → strip trailing commas. Error string on total failure (no silent {}).
§9.9	Tool Execution Error Safety	MUST	Catch exceptions/panics from tool handlers, convert to error string, emit error event, continue loop.
§9.10	LLM Call Retry	SHOULD	Retry execute_llm on transient errors (429, 5xx) with exponential backoff min(2^n + jitter, 60). ExecuteError carries conversation messages for resume.
§12.5	Executor Retry	SHOULD	Executors retry HTTP internally (transparent to loop).

Also updated: §9.1 constants (MAX_LLM_RETRIES), §9.2 algorithm pseudocode, §12.4 error table, §13.7 unified signature.

Runtime Implementations

Feature	Rust	Python	TypeScript	C#
§9.8 Resilient Parsing	✅	✅	✅	✅
§9.9 Tool Error Safety	✅	✅	✅	✅
§9.10 LLM Retry	✅	✅	✅	✅

Test Results

Runtime	Unit Tests	Integration Tests
Rust	237 ✅	9/10 ✅ (1 pre-existing structured output test issue)
Python	910 ✅	48/48 ✅
TypeScript	611 ✅	21/21 ✅
C#	735 ✅	273/275 ✅ (2 skipped — Entra ID perms)

Bug Fixes Applied During Review

• TypeScript: Backoff formula used milliseconds instead of seconds
• Rust: Backoff sleep wasn't cancellable; jitter applied outside cap (max 61s → should be 60s)

Documentation Updates

• specification/agent-loop.mdx — Added §9.8, §9.9, §9.10 sections; updated §9.1 and §9.2
• core-concepts/agent-mode.mdx — Rewrote "Error Recovery & Resilience" with examples in all 4 languages
• implementation/{python,typescript,csharp,rust}.mdx — Added resilience bullet points and max_llm_retries to code samples

Error Types

All runtimes expose an ExecuteError that carries the conversation messages, enabling callers to resume failed agent loops:

# Python
except ExecuteError as e:
    resume_messages = e.messages

# TypeScript
catch (e) { if (e instanceof ExecuteError) resume = e.messages; }

# C#
catch (ExecuteError e) { var resume = e.Messages; }

// Rust
Err(InvokerError::ExecuteRetryExhausted { messages, .. }) => { /* resume */ }