docs: refresh tmuxicate project overview

[coyaSONG]

What changed

• rewrite the README around observable, durable multi-agent coordination
• document the implemented coordinator, review, blocker, target, and picker commands
• correct the Go requirement to match go.mod
• add an architecture diagram, reliability model, current capabilities, and CI-aligned development commands
• add the MIT license file already declared by the project

Why

The previous README still described implemented commands as planned v0.2 work and advertised Go 1.24+ even though the module requires Go 1.26.1. The new overview matches the shipped CLI and makes the file-backed trust model visible above the fold.

Validation

• make build
• make test — all packages passed with the race detector
• golangci-lint v2.11.4 run ./... — 0 issues
• git diff --check