fix(agents): prevent user/project .md agents from overriding builtin agent modes

When users have .md agent files in ~/.claude/agents/ with the same names
as builtin agents (e.g. sisyphus.md, hephaestus.md, atlas.md),
loadAgentsFromDir() hardcodes mode: "subagent" for all loaded agents.

Because the config assembly spreads userAgents after builtinAgents:

  config.agent = {
    ...builtinAgents,  // sisyphus: mode="primary"
    ...userAgents,     // sisyphus: mode="subagent" ← overrides
  }

this causes all primary agents to become subagents. The TUI filters out
subagents, so only non-plugin agents (like "docs") appear in the agent
selector.

Fix:
- Filter out user/project agents that share names with builtin agents
  before spreading into config.agent (both sisyphus-enabled and fallback
  branches)
- Respect frontmatter `mode` field in .md agent files instead of
  hardcoding "subagent"
- Add `mode` to AgentFrontmatter type

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/features/claude-code-agent-loader/loader.ts

@@ -44,7 +44,7 @@ function loadAgentsFromDir(agentsDir: string, scope: AgentScope): LoadedAgent[]
 
        const config: AgentConfig = {
          description: formattedDescription,
-         mode: "subagent",
+         mode: data.mode || "subagent",
          prompt: body.trim(),
        }