feat: add tmux runtime adapter

[harshitsinghbhandari]

Summary

• add a real tmux implementation of ports.Runtime
• validate tmux session IDs and use the AO session ID as the tmux session name
• wrap launch commands with env exports, PATH preservation, and an interactive shell keep-alive
• support short send-keys input and buffer-paste for multiline/long messages
• add unit-tested command builders plus a tmux integration test that skips when tmux is unavailable

Scope

This PR intentionally implements only the tmux Runtime adapter. git-worktree Workspace is left for follow-up per Harshit direction.

Refs #9

Validation

From backend/:

• gofmt -l .
• go build ./...
• go vet ./...
• go test -race ./...

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Pull request overview

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