feat(agents): custom models for sub-agents (forge, sage, muse)

[chengyixu]

Summary

Closes #2637

Adds per-agent model and provider configuration so users can assign different models to each built-in agent. For example, use an expensive thinking model like Opus for forge while using a fast/cheap model like MiniMax via OpenRouter for sage research.

New ZSH commands

Command	Alias	Description
:config-forge-model	:cfm	Set model for the forge (implementation) agent
:config-sage-model	:csgm	Set model for the sage (research) agent
:config-muse-model	:cmm	Set model for the muse (planning) agent

CLI commands

# Set per-agent model
forge config set agent-model forge open_router anthropic/claude-3.5-sonnet
forge config set agent-model sage open_router minimax/minimax-01

# Get per-agent model
forge config get agent-model sage
# Output:
# open_router
# minimax/minimax-01

How it works

• When switching agents (:forge, :sage, :muse), the prompt automatically shows the agent's configured model
• Per-agent overrides take precedence over the global default model
• If no per-agent override is set, the agent falls back to the global model (existing behavior)
• :model and :config-model remain scoped to the global default, as described in the issue
• :config list now shows per-agent model overrides

Implementation

Full-stack changes across 14 files:

• Domain (forge_domain): AgentModelConfig { provider, model } struct, AppConfig.agent_models HashMap, SetAgentModel/ClearAgentModel operations
• Config (forge_config): forge_model, sage_model, muse_model fields persisted to .forge.toml
• Repository (forge_repo): Bidirectional mapping between TOML config and domain types
• Services (forge_services): get/set/clear_agent_model_config methods; agent registry applies overrides at load time
• API (forge_api): Exposes per-agent config via the API trait; invalidates agent cache on changes
• CLI (forge_main): forge config set/get agent-model subcommands; :config list shows overrides
• ZSH plugin: Three new action handlers reusing _forge_pick_model for consistent fzf UX

Config file example

After configuring, .forge.toml will contain:

[forge_model]
provider_id = "open_router"
model_id = "anthropic/claude-3.5-sonnet"

[sage_model]
provider_id = "open_router"
model_id = "minimax/minimax-01"

Test plan

• All existing tests pass (239 forge_main, 262 forge_repo, 198 forge_services, all domain/app/api/config tests)
• cargo check succeeds
• Manual test: :config-sage-model opens fzf picker, selection persists to .forge.toml
• Manual test: :config-forge-model and :config-muse-model work identically
• Manual test: switching agents shows correct per-agent model in prompt
• Manual test: forge config set agent-model sage open_router minimax/minimax-01 works
• Manual test: forge config get agent-model sage returns the configured values
• Manual test: :config shows per-agent overrides in the configuration list

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