Reframe naming: interactive-tmux -> multi-agent workspace (tmux = optional feature, not identity)

[NeuralEmpowerment]

Summary

The interactive-tmux naming misleads: it implies the workspace IS a tmux thing,
when tmux is just ONE optional feature (interactive steering) riding on what is
really a multi-agent execution workspace (claude + codex + gemini CLIs baked in).
The default/primary execution path is now headless docker-exec (DockerExecExecutor,
codex exec / claude -p, exit-code completion) - NOT tmux. The naming should reflect
that; tmux becomes a named feature, not the identity.

This caused real confusion (a reader concluded "we're going back to tmux" purely from
the naming, when the recommended path is headless). We own agentic-primitives, so
dogfood the fix.

Rename / reframe (harness-neutral)

• Docker image agentic-workspace-interactive-tmux -> a multi-agent name (e.g.
  agentic-multi-agent-workspace or agentic-workspace). tmux = a feature inside it.
• Crate/binary itmux (interactive-tmux) -> consider a harness-neutral name
  (e.g. agentrun / harness). Higher surface (binary name flows into run_client.py
itmux_bin, syn137 image, docs) - may stage this separately or defer.
• Module path providers/workspaces/interactive-tmux/ -> reflect multi-agent.
• syn137 tmux-isms (separate but related): SYN_WORKSPACE_INTERACTIVE_TMUX_* env,
  the claude-interactive provider value, ITMUX_*_HOME - rename harness-neutral.

Context

• Default execution is headless docker-exec (--backend docker-exec); the tmux driver
  stays as an off-by-default optional feature (--backend tmux), parked per syn137#777.
• This is item chore: delete temp plan #5 of the old "harness-neutral substrate" goal (rename off tmux-isms).
• Priority: low / cleanup - "whatever for now" per the owner; do it as we touch these
  areas building the docker-exec RunExecutor, not as a blocking refactor.

Refs

• syn137#777 (park interactive-tmux execution path)
• syn137#778 (build-it-right: codex/claude via AP RunExecutor)
• DockerExecExecutor plan (AP worktree 20260722_docker-exec-runexecutor)