Summary

• When call_llm_for_session_summary() times out waiting for the structured JSON output call, fall through to the plain-text chat() fallback instead of returning None and silently dropping the session summary.
• Extract plain_text_summary_fallback() helper to share fallback logic between the Ok(Err(e)) (error) and Err(_) (timeout) arms.
• Add shutdown_summary_timeout_secs (default: 10) to [memory] config, replacing the hardcoded 5s limit.

Root Cause

Err(_) => {
    warn!("shutdown summary: LLM call timed out after 5s");
    None  // ← no fallback attempted
}

The plain-text fallback was only reached on Ok(Err(e)), not on timeout.

Changes

• crates/zeph-core/src/agent/mod.rs: timeout arm now calls plain_text_summary_fallback(); helper extracted from duplicated Ok(Err(e)) path
• crates/zeph-core/src/config/types/memory.rs: shutdown_summary_timeout_secs: u64 (default: 10)
• crates/zeph-core/src/agent/builder.rs, context/mod.rs, src/runner.rs: wired through
• crates/zeph-core/src/agent/tests.rs: all call sites updated, 3 new assertions
• Insta snapshots updated for both snapshot variants

Test plan

• cargo +nightly fmt --check — clean
• cargo clippy --workspace --features full -- -D warnings — clean
• cargo nextest run --workspace --features full --lib --bins — 5971 passed, 12 skipped

Closes #1869