refactor: remove builtin agent runtime and pinix chat TUI

[yan5xu]

Summary

Remove the Go Agent Runtime, TUI, and pinix chat command from pinixd.

Agent functionality is now exclusively provided by the cloud service at pinixai.com via the /ai/* API endpoints in cmd/api.

What's removed

Code (~4700 lines deleted)

• internal/agent/ (9 files) — LLM client, tool loop, context manager, SQLite store
• internal/builtin/agent.go — agent builtin clip bridge
• internal/tui/ (3 files) — bubbletea v2 TUI for pinix chat
• cmd/pinix/chat.go — pinix chat cobra command

Modifications

• internal/daemon/daemon.go — removed initAgentRuntime(), listAgentClips(), daemonClipInvoker, and agent imports
• cmd/pinix/main.go — removed newChatCommand() registration
• go.mod/go.sum — removed unused dependencies (bubbletea, LLM SDKs, SQLite driver, etc.)

Documentation

• AGENTS.md — removed agent/tui/chat references from source-of-truth, directory structure, and dev commands
• docs/concepts.md — updated agent-clip references to generic cloud agent
• docs/protocol.md — replaced agent examples with todo clip examples
• docs/vision.md — updated to reference cloud agent at pinixai.com

What's NOT removed

• internal/builtin/builtin.go — framework code (unchanged)
• internal/builtin/scheduler.go — scheduler builtin clip (unchanged, no agent dependency)
• internal/daemon/scheduler.go — scheduler (independent, no agent dependency)

Verification

• go build ./cmd/pinixd — OK
• go build ./cmd/pinix — OK
• go vet ./... — OK
• go mod tidy — cleaned