Replace anyhow with typed AgentError in zeph-core agent module

[bug-ops]

Problem

zeph-core/src/agent/ modules (mod.rs, context.rs, learning.rs, mcp.rs, streaming.rs, persistence.rs) use anyhow::Result throughout. While project conventions allow anyhow in application code, the agent module has grown into a substantial orchestration layer with well-defined error categories that would benefit from typed errors.

Proposed solution

Introduce AgentError enum in zeph-core/src/agent/error.rs with variants covering all failure modes:

• LLM errors (LlmError from zeph-llm)
• Channel errors (ChannelError)
• Memory/persistence errors (MemoryError)
• Skill errors (SkillError)
• Tool execution errors
• Configuration errors
• Timeout errors

Replace anyhow::Result in agent methods with Result<T, AgentError>. Keep anyhow only in main.rs entry point.

Affected files

• crates/zeph-core/src/agent/mod.rs
• crates/zeph-core/src/agent/context.rs
• crates/zeph-core/src/agent/learning.rs
• crates/zeph-core/src/agent/mcp.rs
• crates/zeph-core/src/agent/streaming.rs
• crates/zeph-core/src/agent/persistence.rs
• New: crates/zeph-core/src/agent/error.rs

Part of #282