Add config default for swarm spawn mode

[Zephyr709]

Background

PR #209 adds per-call spawn_mode = visible | headless | auto for swarm-created agents so callers can avoid focus-stealing headed terminals.

Follow-up

Add a user/global config default so users can prefer headless swarm workers without specifying spawn_mode on every call.

Possible shape:

[agents]
swarm_spawn_mode = "headless" # visible | headless | auto

Explicit tool-call spawn_mode should override the config default.

Acceptance criteria

• Config parses and validates visible, headless, and auto.
• Omitted tool spawn_mode uses the config default.
• Existing behavior remains unchanged when no config is set.
• Tests cover defaulting and explicit override.

[1jehuang]

Implemented in e354a3d.

Summary:

• Added [agents].swarm_spawn_mode with supported values visible, headless, and auto.
• Kept the default as visible to preserve existing headed/visible behavior when no config is set.
• Added per-call spawn_mode support for the swarm tool, with explicit tool input overriding the config default.
• Added parsing/validation coverage and protocol/schema tests.

Validation run:

• cargo check -q -p jcode-config-types -p jcode-protocol -p jcode --bin jcode
• cargo test -q -p jcode swarm_spawn_mode -- --test-threads=1
• cargo test -q -p jcode schema_advertises_supported_swarm_fields
• cargo test -q -p jcode-protocol test_comm_spawn_roundtrip_with_optional_nonce
• cargo build --profile selfdev -p jcode --bin jcode

[1jehuang]

Closing as implemented in e354a3d.