Configurable iteration cap with doom-loop detection

[bug-ops]

Parent: #239 (Phase B)

Summary

Replace hardcoded MAX_SHELL_ITERATIONS = 3 with configurable limit (default 10) and add doom-loop detection.

Changes

• crates/zeph-core/src/config.rs: add max_tool_iterations: usize to AgentConfig (default 10)
• crates/zeph-core/src/agent.rs: use config value instead of constant
• Doom-loop detection: track last N tool outputs, break on 3 consecutive identical outputs/errors, notify user
• Per-iteration token budget check (stop if approaching context limit)
• config/default.toml: add [agent] max_tool_iterations = 10
• Env var override: ZEPH_AGENT_MAX_TOOL_ITERATIONS

Acceptance criteria

• Iteration limit configurable via TOML + env var
• Default raised from 3 to 10
• Doom-loop detection breaks on 3 consecutive identical outputs
• Agent stops when context budget nears limit
• Existing tests updated