feat(settings): add includeGitInstructions toggle

Adds an 'Include git instructions' toggle to the Agent Settings panel.
When disabled, sets CLAUDE_CODE_DISABLE_GIT_INSTRUCTIONS=1 on the Claude
process to remove built-in commit and PR workflow guidance from its
system prompt. Resolves #209.

src/lib/commands/slashCommands.test.ts

@@ -65,6 +65,7 @@ vi.mock("$lib/stores/config", () => ({
       use_worktree: false,
       disable_1m_context: false,
       max_output_tokens: null,
+      include_git_instructions: true,
     }),
   },
 }));