Closes #2342.

Motivation

Agents invoked by automated sources (cron ticks, event triggers, agent_send) currently always reuse the agent's persistent session. This means context accumulates indefinitely across background invocations — fine for stateful assistants, but wrong for stateless jobs like "summarize today's emails" or "run a health check" where each invocation should start clean.

This PR adds a session_mode knob so agent authors can opt into fresh sessions per invocation without changing any existing behavior.

Changes

Types (librefang-types/src/agent.rs)

• New SessionMode enum (Persistent | New) with #[serde(rename_all = "snake_case")], Default = Persistent.
• New session_mode: SessionMode field on AgentManifest with #[serde(default)] — backward-compatible, existing TOML files parse unchanged.

Trigger engine (librefang-kernel/src/triggers.rs)

• New session_mode: Option<SessionMode> on Trigger — per-trigger override, None inherits from the target agent's manifest.
• New TriggerMatch struct replaces the old (AgentId, String) tuple returned by evaluate(), carrying agent_id, message, and session_mode_override through dispatch.
• All existing tests updated from tuple destructuring to TriggerMatch field access.

Kernel dispatch (librefang-kernel/src/kernel.rs)

• send_message_full gains a session_mode_override: Option<SessionMode> parameter (all existing call sites pass None).
• New private send_message_with_session_mode — used exclusively by trigger dispatch to plumb per-trigger overrides without changing public API surface.
• Session ID resolution in both send_message_full and execute_llm_agent:
  • Channel-derived sessions are unaffected — always deterministic per-channel.
  • Non-channel invocations resolve: per-trigger override > agent manifest session_mode > Persistent (default).
  • SessionMode::New → SessionId::new() (fresh UUID each time).
• publish_event now iterates Vec<TriggerMatch> and calls send_message_with_session_mode.

Wizard (librefang-kernel/src/wizard.rs)

• Setup wizard sets session_mode: SessionMode::default() on generated manifests.

Docs

• AGENTS.md: documents the session_mode field and per-trigger overrides.
• CHANGELOG.md: added under [Unreleased].
• CLAUDE.md: architecture note on session resolution.

Scope

Intentionally does not:

• Add API routes for toggling session_mode at runtime — it's a manifest/TOML field, changed by editing agent.toml and reloading.
• Affect hands differently — hands share AgentManifest and the execution pipeline, so session_mode works automatically.
• Touch channel-derived sessions — those remain deterministic per-channel regardless of session_mode.

Test plan

• All existing trigger tests updated to TriggerMatch field access and pass
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo fmt --check clean
• Create agent with session_mode = "new", verify each cron/trigger invocation starts a fresh session (new session ID in logs)
• Create agent with default config, verify persistent session behavior unchanged
• Register trigger with session_mode = "new" override on an agent that defaults to "persistent", verify override wins
• Send message via channel (Telegram/Discord) to a session_mode = "new" agent, verify channel-derived session is used (not a fresh one)