Implement HC014: Missing Entrypoint detection

## Overview
Detects when no valid entrypoint (Main.main or main) exists in analyzed code.

## Implementation Details
- **Rule**: `rule_missing_entrypoint.hako` following single-responsibility box principles
- **Detection**: Checks if any entrypoint from entrypoints[] exists in methods[]
- **Pattern Matching**: Matches "Main.main" or "main" with any arity (e.g., Main.main/0, Main.main/1)
- **Integration**: Added to cli.hako with debug output support

## Test Cases
- **ok.hako**: Main box with main() method → no warning
- **ng.hako**: Main box with run() method (not main) → HC014 + HC011 warnings
  - HC011: Main.run/0 unreachable (no entrypoint calling it)
  - HC014: Missing entrypoint (correct cascading diagnostics)

## Test Results
```
[TEST/OK] HC011_dead_methods
[TEST/OK] HC012_dead_static_box
[TEST/OK] HC013_duplicate_method
[TEST/OK] HC014_missing_entrypoint ← NEW
[TEST/OK] HC016_unused_alias
[TEST/SUMMARY] all green
```

## Architecture
- Box-first design: RuleMissingEntrypointBox with single responsibility
- Helper method: _has_entrypoint_method() for clean separation of concerns
- Diagnostic format: "[HC014] missing entrypoint (Main.main or main)"
- Severity: "warning" (non-blocking, informational)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tools/hako_check/tests/HC014_missing_entrypoint/expected.json

@@ -0,0 +1,4 @@
+{"diagnostics":[
+  {"file":"ng.hako","line":1,"rule":"HC011","message":"[HC011] unreachable method (dead code): PLACEHOLDER :: Main.run/0","quickFix":"Remove or reference the dead method from an entrypoint","severity":"error"},
+  {"file":"ng.hako","line":1,"rule":"HC014","message":"[HC014] missing entrypoint (Main.main or main)","quickFix":"","severity":"warning"}
+]}