Make workspace context reflect real git state

Git-aware CLI flows already existed, but branch detection depended on
status-line parsing and /diff hid local policy inside a path exclusion.
This change makes branch resolution and diff rendering rely on git-native
queries, adds staged+unstaged diff reporting, and threads git diff
snapshots into runtime project context so prompts see the same workspace
state users inspect from the CLI.

Constraint: No new dependencies for git integration work
Constraint: Slash-command help/behavior must stay aligned between shared metadata and CLI handlers
Rejected: Keep parsing the `## ...` status line only | brittle for detached HEAD and format drift
Rejected: Keep hard-coded `:(exclude).omx` filtering | redundant with git ignore rules and hides product policy in implementation
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Preserve git-native behavior for branch/diff reporting; do not reintroduce ad hoc ignore filtering without a product requirement
Tested: cargo fmt; cargo clippy --workspace --all-targets -- -D warnings; cargo test --workspace
Not-tested: Manual REPL /diff smoke test against a live interactive session

rust/crates/runtime/src/conversation.rs

@@ -408,12 +408,13 @@ mod tests {
                 .sum::<i32>();
             Ok(total.to_string())
         });
-        let permission_policy = PermissionPolicy::new(PermissionMode::Prompt);
+        let permission_policy = PermissionPolicy::new(PermissionMode::WorkspaceWrite);
         let system_prompt = SystemPromptBuilder::new()
             .with_project_context(ProjectContext {
                 cwd: PathBuf::from("/tmp/project"),
                 current_date: "2026-03-31".to_string(),
                 git_status: None,
+                git_diff: None,
                 instruction_files: Vec::new(),
             })
             .with_os("linux", "6.8")
@@ -487,7 +488,7 @@ mod tests {
             Session::new(),
             SingleCallApiClient,
             StaticToolExecutor::new(),
-            PermissionPolicy::new(PermissionMode::Prompt),
+            PermissionPolicy::new(PermissionMode::WorkspaceWrite),
             vec!["system".to_string()],
         );
 
@@ -536,7 +537,7 @@ mod tests {
             session,
             SimpleApi,
             StaticToolExecutor::new(),
-            PermissionPolicy::new(PermissionMode::Allow),
+            PermissionPolicy::new(PermissionMode::DangerFullAccess),
             vec!["system".to_string()],
         );
 
@@ -563,7 +564,7 @@ mod tests {
             Session::new(),
             SimpleApi,
             StaticToolExecutor::new(),
-            PermissionPolicy::new(PermissionMode::Allow),
+            PermissionPolicy::new(PermissionMode::DangerFullAccess),
             vec!["system".to_string()],
         );
         runtime.run_turn("a", None).expect("turn a");