ADR 0198: Worktree port export is opt-in

Status: accepted

Every discern worktree has a deterministic development port as part of its identity. The port is always available through discern identity --port and the @port@ setup token. The [worktree].port setting has a narrower effect: it writes DISCERN_WORKTREE_PORT into a configured env file.

Fresh configs enabled that write even when a project had no tooling that read the variable. This made a derived identity value look like a provisioned resource and introduced env-file churn for projects that did not need it.

Fresh configs ship with [worktree].port = false. A project enables it when its development tooling consumes DISCERN_WORKTREE_PORT.

The setting controls only env-file export. Deterministic port derivation, discern identity --port, and @port@ remain available while it is false. Schema 23 does not rewrite the setting in existing installs.

• A fresh worktree does not write an unused port variable into an env file.
• Projects with concurrent dev servers opt in with one explicit setting.
• Existing installs keep their current env-file behaviour.
• Documentation must distinguish port identity from port export.

• Keep export on by default. Rejected because most projects do not consume the variable, while the identity remains discoverable without exporting it.
• Remove the setting and always derive on demand. Rejected because env-file export is useful to project tooling that cannot invoke discern directly.