Restore research as first-class tool, remove research skill

Restores the research tool that was previously externalized as a skill:

- Add pending_research.rs: PendingResearchManager with thread-safe task tracking
- Add tools/research.rs: execute_research (async), execute_research_status
- Add research/research_status tool definitions with exclude_research config
- Integrate PendingResearchManager into Agent and ToolContext
- Inject completed research results in streaming loop

Remove research skill:
- Clear EMBEDDED_SKILLS array in embedded.rs
- Delete skills/research/ directory
- Update all tests expecting embedded research skill
- Update docs and memory to reflect the change

The research tool now:
- Spawns scout agent in background tokio task
- Returns immediately with research_id
- Automatically injects results into conversation when ready
- Supports status checks via research_status tool

docs/architecture.md

@@ -259,14 +259,13 @@ Key modules in `crates/g3-core/src/skills/`:
 Core skills are embedded at compile time using `include_str!`, ensuring g3 works anywhere without external files:
 
 ```rust
+// Currently empty - skills can be added here as needed
 static EMBEDDED_SKILLS: &[EmbeddedSkill] = &[
-    EmbeddedSkill {
-        name: "research",
-        skill_md: include_str!("../../../../skills/research/SKILL.md"),
-        scripts: &[
-            ("g3-research", include_str!("../../../../skills/research/g3-research")),
-        ],
-    },
+    // Example:
+    // EmbeddedSkill {
+    //     name: "example-skill",
+    //     skill_md: include_str!("../../../../skills/example-skill/SKILL.md"),
+    // },
 ];
 ```