Migrate runtime adapter from Zellij → tmux

[i-trytoohard]

Migrate runtime adapter from Zellij → tmux

Problem

The backend runtime is currently hardwired to Zellij. The daemon imports and instantiates zellij.Runtime directly with no abstraction layer:

• backend/internal/daemon/daemon.go:16 — imports adapters/runtime/zellij
• backend/internal/daemon/daemon.go:99 — creates runtimeAdapter := zellij.New(zellij.Options{...})
• backend/internal/daemon/daemon.go:89-96 — zellij-specific socket dir handling (DefaultSocketDir(), ZELLIJ_SOCKET_DIR)
• backend/internal/daemon/lifecycle_wiring.go:13 — imports zellij package
• backend/internal/daemon/lifecycle_wiring.go:65 — startSession takes runtime *zellij.Runtime (concrete type, not interface)
• backend/internal/daemon/lifecycle_wiring.go:130 — comments reference zellij.Runtime.SendMessage

The ports.Runtime interface (backend/internal/ports/outbound.go:79) only requires Create, Destroy, IsAlive — the contract is clean, but the wiring bypasses it by depending on the concrete zellij.Runtime type.

Goal

Replace Zellij with tmux as the terminal multiplexer runtime. This:

• Removes a hard dependency on Zellij (less common install base than tmux)
• Eliminates the macOS socket-path-length workaround (DefaultSocketDir)
• Aligns with the upstream fork (aoagents/ReverbCode) which already completed this migration in #404

Scope

1. New adapter: backend/internal/adapters/runtime/tmux/ implementing ports.Runtime — tmux equivalents of Create (new-session), Destroy (kill-session), IsAlive (list-sessions), plus Attach/Stream/SendMessage.
2. Abstract the wiring: Introduce a runtimeselect (or similar) layer so daemon.go and lifecycle_wiring.go depend on ports.Runtime, not the concrete type. startSession should take ports.Runtime (or runtimeselect.Runtime), not *zellij.Runtime.
3. Remove zellij-specific code: socket dir handling, DefaultSocketDir(), zellij config/kdl handling, min-version checks (minMajor/minMinor/minPatch).
4. Update tests: All zellij_test.go references, fake runner patterns, etc.
5. Docs: Update any references to zellij in docs/ and comments.

Reference

ReverbCode PR #404 "Zellij to Tmux and some other fixes" is a complete prior implementation of this exact migration. The tmux adapter there (backend/internal/adapters/runtime/tmux/tmux.go) can be used as the blueprint.

[somewherelostt]

@harshitsinghbhandari i checked and we have some hardcoded zellij !