feat(commands): Session management [Phase 3/3] command execution centralization

[mingmxren]

📝 Description

• Centralizes command execution with tri-state executor/runtime contracts.
• Routes generic command handling through one runtime-backed path while preserving /switch passthrough behavior.
• Removes dead parser/dispatcher remnants and keeps channel behavior aligned.
• Removes docs/plans artifacts from this PR and moves the high-level implementation rationale into English comments in key executor/runtime/agent functions.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

Checked on 2026-03-03 in sipeed/picoclaw upstream issues:

• [Feature] Unify slash commands across all channels #570 [Feature] Unify slash commands across all channels
• [BUG] Telegram commands bypass agent loop — missing feature parity #583 [BUG] Telegram commands bypass agent loop — missing feature parity (closed)

📚 Technical Context (Skip for Docs)

• Reference URL: code comments in pkg/commands/executor.go, pkg/commands/builtin.go, pkg/agent/loop.go
• Reasoning: A single command execution path reduces channel drift, preserves reject/passthrough semantics, and keeps command behavior deterministic.

🧪 Test Environment

• Hardware: Mac mini
• OS: macOS 26.3
• Model/Provider: gpt-5.2, gemini-3-flash-preview, gemini-3-pro-preview
• Channels: Telegram, WhatsApp

📸 Evidence (Optional)

Click to view Logs/Screenshots

• go test ./pkg/commands ./pkg/agent ./pkg/session ./pkg/config ./pkg/channels/...

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

[mingmxren]

Superseded by #959 after merging Phase 3 changes into PR1. We now keep a two-branch train with #959 first and #960 follow-up.