fix(config): add deep_research flag to gate Sage agent

[tusharmath]

Summary

Add a deep_research feature flag to gate the Sage agent behind a config option, keeping it disabled by default.

Context

The Sage agent provides deep research capabilities but is not yet ready for general availability. This change introduces a configuration flag so the Sage agent (and its :sage app command) can be selectively enabled without requiring a code change or a separate build.

Changes

• Added deep_research: bool field to ForgeConfig (defaults to false)
• Updated the default .forge.toml config to include deep_research = false
• Updated forge.schema.json to document the new field
• Implemented filter_agent in the agent repository to exclude the Sage agent from get_agents and get_agent_infos when the flag is disabled
• Applied the filter to both AgentRepository::get_agents and AgentRepository::get_agent_infos

Key Implementation Details

A single filter_agent function checks whether the agent id matches AgentId::SAGE and, if so, gates it on config.deep_research. All other agents pass through unconditionally. The config is read once per call and shared between the filter and the rest of the pipeline to avoid redundant reads.

Use Cases

• Users on a stable release who do not need deep research get a clean agent list with no Sage agent present
• Power users or teams that want deep research can opt in by setting deep_research = true in their .forge.toml
• The flag provides a safe toggle for gradual rollout without shipping a separate binary

Testing

# Run all tests
cargo nextest run

# Verify the filter works by setting deep_research = false (default) and
# checking that the sage agent does not appear in the agent list
# Then set deep_research = true and verify it appears
cargo insta test --accept

Links

• Related to the Sage agent feature (AgentId::SAGE)

[tusharmath]

User should be able to switch to sage if they want to. Sage should only be disabled as a sub-agent.

[tusharmath]

Related to #3076