Decision 056: tmux is the only runner — the headless process runner is removed

• Status: proposed
• Date: 2026-08-05
• Deciders: @MadaraUchiha-314 (issue #156 comment)
• Work item: issue-156
• Spec: docs/specs/issue-156/

Context

Since decision-021, routing.runner chose between two ways of hosting a daemon-driven session: the original headless one-shot subprocess of decision-016 (claude -p … --resume, default) and the attachable tmux-hosted TUI. The choice was made at first spawn and then copied into the session record, and every later dispatch trusted the copy ("the session's recorded runner wins", mixed fleets). Issue #156 showed the cost: a record whose runner field says "process" — the silent dataclass default, written by every sessions register — reroutes all future events for that work item to an invisible subprocess while the operator watches an idle tmux pane. No reconciliation, no warning; the failure mode is silence.

Decision

Remove the process runner entirely (owner decision on the ticket):

• Every daemon-spawned session is hosted in a named tmux session (loop-<slug>); tmux becomes a required dependency of gh-webhook start / poll start.
• routing.runner leaves the config schema; a leftover key is ignored with a warning, never an error.
• The session record's runner field is removed; a legacy record carrying one parses fine and the key is not read. A record with no tmuxTarget (legacy process records, sessions register self-registrations) heals lazily: its next dispatched event takes the existing respawn path (issues 80/89/146) and resumes the recorded conversation in a fresh tmux session when possible.
• The harness adapter contract loses its headless per-dispatch spawn/resume surface. The one-shot invocation used by critic reviews (oneshot_argv, decision-043) is explicitly retained — it is a review mechanism, not a session runner — as are the interactive tmux methods.

Consequences

• The issue's defect class is structurally gone: with one runner there is no per-record selector to go stale and no silent execution path to fall into.
• Supersedes the runner choice of decision-021 (whose tmux design otherwise stands in full) and the headless per-event dispatch of decision-016 (whose CLI-not-MCP and adapter-contract decisions stand; the CLI is now invoked interactively under tmux, or one-shot for critics).
• Mixed fleets (per-work-item runner selection, deferred by decision-021) are off the table by construction.
• cursor-agent cannot pre-assign an interactive session id, so it cannot host daemon sessions; it remains a critic harness. Interactive cursor support is a future adapter change, not a config toggle.