fix(delegate): respect chat.defaultAgent setting when no agent is specified

[sandikodev]

Issue

Closes #3174

Problem

When the delegate tool is invoked without an explicit agent parameter, it unconditionally falls back to the hardcoded DEFAULT_AGENT_NAME ("q_cli_default"), ignoring the user's chat.defaultAgent setting.

Reproduction:

q settings chat.defaultAgent my-agent
q chat
# delegate a task without specifying an agent
# → uses q_cli_default instead of my-agent

Fix

Change the fallback chain from:

// Before: always falls back to hardcoded default
let agent_name = self.agent.as_deref().unwrap_or(DEFAULT_AGENT_NAME);

To:

// After: respects chat.defaultAgent setting
let configured_default = os.database.settings.get_string(Setting::ChatDefaultAgent);
let agent_name = self
    .agent
    .as_deref()
    .or(configured_default.as_deref())
    .unwrap_or(DEFAULT_AGENT_NAME);

New fallback chain:

1. Explicitly provided agent parameter (unchanged)
2. chat.defaultAgent setting (if configured)
3. DEFAULT_AGENT_NAME as last resort

Testing

# Run the targeted tests
cargo test -p chat_cli 'delegate::tests'

# Run the full test suite to check for regressions
cargo test -p chat_cli

Four tests cover the full fallback chain:

1. explicit_agent_takes_priority — explicit parameter overrides everything
2. configured_default_used_when_no_explicit_agent — chat.defaultAgent is used when no explicit agent is given
3. falls_back_to_default_agent_name_when_nothing_configured — DEFAULT_AGENT_NAME is used as last resort
4. get_schema — existing schema test (unchanged)

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[sandikodev]

Update (amended commit): Tests were added after initial submission to verify the fallback chain logic:

1. Explicit agent parameter takes priority over everything
2. chat.defaultAgent setting is used when no explicit agent is given
3. DEFAULT_AGENT_NAME is used as last resort when nothing is configured

The resolution logic was extracted into a local resolve_agent_name helper inside the test module to make it directly testable without requiring a full Os + Agents setup.