fix: implement preset 'replaces' field to remove superseded commands (v0.1.3)

[kanfil]

Summary

Implements the replaces field in preset manifest commands, which was defined but not functional.

Problem

When installing the agentic-sdlc preset, both old (speckit.*) and new (adlc.spec.* + spec.*) command files were present:

.opencode/command/
├── speckit.specify.md      # Old - should be removed
├── speckit.plan.md         # Old - should be removed
├── adlc.spec.specify.md    # New - from preset
├── spec.specify.md         # New - alias from preset
├── adlc.spec.plan.md       # New - from preset
├── spec.plan.md            # New - alias from preset
...

Solution

1. Added remove_replaced_commands() method to CommandRegistrar in agents.py
2. Updated _register_commands() in presets.py to:
   • Collect all replaces values from command templates
   • Remove those files from all detected agent directories
   • Then write the new commands and aliases
3. Fixed missing replaces: "speckit.constitution" in preset.yml

After fix

.opencode/command/
├── adlc.spec.specify.md    # New command
├── spec.specify.md         # Alias
├── adlc.spec.plan.md       # New command
├── spec.plan.md            # Alias
├── speckit.taskstoissues.md  # No replacement defined - stays
...

Testing

• All 356 pytest tests pass
• Manual test: specify init --ai opencode --here produces correct command set