feat(a2a,config): replace EchoTaskProcessor, reconcile vault config

[bug-ops]

Summary

• feat(a2a): replace EchoTaskProcessor with real agent bridge #262 Replace EchoTaskProcessor with AgentTaskProcessor that routes A2A messages through provider.chat() for MVP inference. System prompt includes agent name and skill list.
• fix(config): vault.backend from TOML ignored, only CLI args used #263 Reconcile vault config: config.vault.backend used as default, CLI --vault overrides. Priority: CLI > env > config > "env".
• Sanitize A2A error responses to prevent provider URL/model leakage to external callers.

Test plan

• vault_args_uses_config_backend_as_fallback — config fallback works
• vault_args_env_overrides_config — env var takes priority over config
• agent_task_processor_construction — AgentTaskProcessor wiring validated
• 1310 tests pass, clippy zero warnings

Closes #262, closes #263

[codecov-commenter]

Codecov Report

❌ Patch coverage is 47.14286% with 37 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/main.rs	47.14%	37 Missing ⚠️

❌ Your patch status has failed because the patch coverage (47.14%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
- Coverage   81.24%   81.04%   -0.21%     
==========================================
  Files          81       81              
  Lines       24194    24220      +26     
==========================================
- Hits        19656    19628      -28     
- Misses       4538     4592      +54     

Files with missing lines	Coverage Δ
src/main.rs	65.67% <47.14%> (-3.53%)	⬇️

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