Feature: make sub-agent nicknames configurable via config.toml

Problem

Sub-agent nicknames are currently hardcoded in agent_names.txt. There's no way for users to customize them — you're stuck with the built-in pool.

Proposal

Add two fields to the [agents] section in config.toml:

[agents]
nicknames = "Atlas, Nova, Vega"
nickname_mode = "replace"   # or "append" (default)

• nicknames — comma-separated list of custom names (trimmed, deduplicated).
• nickname_mode — "append" extends the built-in pool; "replace" uses only the custom names. Ignored when nicknames is unset.

Default behavior is unchanged — when nicknames is not set, the built-in list is used as-is.

Reference implementation

A working implementation with tests is at #13022. It touches three files:

• config/mod.rs — new AgentNicknameMode enum, parsing logic, new fields on Config and AgentsToml
• agent/control.rs — agent_nickname_list() reads config instead of being hardcoded
• config.schema.json — updated schema

Tests cover append mode, replace mode, CSV parsing/dedup, and an end-to-end spawn integration test.