feat(repl): Phase 288 P1 - Add CLI entry point

Added --repl / -i flag:
- src/cli/args.rs: clap flag definition (+5 lines)
- src/cli/mod.rs: CliConfig.repl field (+3 lines)
- src/runner/mod.rs: run_repl() loop + stub eval (+71 lines)

Minimal REPL: .help / .exit work, eval stub for P2.

Test results:
  ✅ hakorune --repl starts REPL
  ✅ .help / .exit / .reset commands work
  ✅ File mode regression: 154/154 tests pass

File mode unchanged (0 regressions).

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/cli/mod.rs

@@ -70,6 +70,8 @@ pub struct CliConfig {
     pub macro_expand_child: Option<String>,
     pub dump_expanded_ast_json: bool,
     pub macro_ctx_json: Option<String>,
+    // Phase 288 P1: REPL mode
+    pub repl: bool,
 }
 
 pub use groups::{
@@ -221,6 +223,8 @@ impl Default for CliConfig {
             macro_expand_child: None,
             dump_expanded_ast_json: false,
             macro_ctx_json: None,
+            // Phase 288 P1: REPL mode
+            repl: false,
         }
     }
 }