feat: CLI v2.1.81 features + global CLAUDE.md editor (#263)

## Summary

Implements support for Claude Code CLI v2.1.81 features and adds a global CLAUDE.md editor, closing issues #237, #239, #244, #245, #246, #247, #248, and #262.

### Stream-JSON forward-compatibility (#245, #246, #247, #248)

- **#248** — `output_style` field added to `System` init message; silently accepted for forward-compat
- **#245** — `fast_mode_state` field added to `Result` message; logged at debug level
- **#246** — `model_usage` field added to `Result` message; per-model breakdown logged at debug level
- **#247** — `total_cost_usd` field added to `Result` message; authoritative cost logged at debug level

### New config options (#237, #239, #244)

- **#237** — `bare_mode` config toggle: passes `--bare` to Claude Code, suppressing UI chrome for scripted headless `-p` calls
- **#239** — `show_clear_context_on_plan_accept` toggle: passes `showClearContextOnPlanAccept: false` in `--settings` when disabled
- **#244** — `custom_model_option` text field: sets `ANTHROPIC_CUSTOM_MODEL_OPTION` env var for custom model providers

### Global CLAUDE.md editor (#262)

- New Tauri commands `get_global_claude_md` / `save_global_claude_md` read/write `~/.claude/CLAUDE.md` (creates file + directory if absent)
- New "Global Instructions" section in the Config Sidebar with a textarea and Save button

### Bug fix (pre-existing)

`disable_cron` and `disable_skill_shell_execution` were saved to `HikariConfig` but never passed to `start_claude` invocations — fixed in all 9 call sites. All 3 new config fields are also wired through all 9 call sites.

All changes pass `check-all.sh` (ESLint → Prettier → svelte-check → Vitest → Clippy → cargo test with llvm-cov).

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #263
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/lib/components/StatusBar.svelte

@@ -94,6 +94,9 @@
     auto_memory_directory: null,
     model_overrides: null,
     disable_skill_shell_execution: false,
+    bare_mode: false,
+    show_clear_context_on_plan_accept: true,
+    custom_model_option: null,
   });
 
   let streamerModeActive = $state(false);
@@ -175,11 +178,17 @@
           use_worktree: currentConfig.use_worktree ?? false,
           disable_1m_context: currentConfig.disable_1m_context ?? false,
           max_output_tokens: currentConfig.max_output_tokens ?? null,
+          disable_cron: currentConfig.disable_cron ?? false,
+          disable_skill_shell_execution: currentConfig.disable_skill_shell_execution ?? false,
           include_git_instructions: currentConfig.include_git_instructions ?? true,
           enable_claudeai_mcp_servers: currentConfig.enable_claudeai_mcp_servers ?? true,
           auto_memory_directory: currentConfig.auto_memory_directory || null,
           model_overrides: currentConfig.model_overrides || null,
           session_name: activeConversationForName?.name || null,
+          bare_mode: currentConfig.bare_mode ?? false,
+          show_clear_context_on_plan_accept:
+            currentConfig.show_clear_context_on_plan_accept ?? true,
+          custom_model_option: currentConfig.custom_model_option || null,
         },
       });
 
@@ -337,11 +346,17 @@
           use_worktree: currentConfig.use_worktree ?? false,
           disable_1m_context: currentConfig.disable_1m_context ?? false,
           max_output_tokens: currentConfig.max_output_tokens ?? null,
+          disable_cron: currentConfig.disable_cron ?? false,
+          disable_skill_shell_execution: currentConfig.disable_skill_shell_execution ?? false,
           include_git_instructions: currentConfig.include_git_instructions ?? true,
           enable_claudeai_mcp_servers: currentConfig.enable_claudeai_mcp_servers ?? true,
           auto_memory_directory: currentConfig.auto_memory_directory || null,
           model_overrides: currentConfig.model_overrides || null,
           session_name: null,
+          bare_mode: currentConfig.bare_mode ?? false,
+          show_clear_context_on_plan_accept:
+            currentConfig.show_clear_context_on_plan_accept ?? true,
+          custom_model_option: currentConfig.custom_model_option || null,
         },
       });