hey watch: filter by label and run a command only for that label

[TomFaulkner]

hey watch already does the right thing for agents: a long-running process, JSON lines as mail lands, --events new, --box, --run-async / --run-sync.

What is missing is a way to subscribe to a label, not a whole box.

Why

HEY labels are the natural whitelist. A contact (or a human in the TUI) applies agent-trades / agent-church; an agent should wake only then. Watching the whole Imbox and filtering after the fact still burns a wake (or a script invocation) on every new posting.

Plus-addresses (you+label@hey.com) auto-label, but many forms reject +, so label-on-contact / manual label is the reliable gate.

This is not a request for IMAP, HTTP webhooks from HEY's servers, or third-party clients. Local hey watch already exists; it just cannot select a label the way it selects --box.

Proposal

hey watch --label 789 --events new
hey watch --label 789 --events new --run-async './wake-agent.sh'
hey watch --label agent-trades --events new --run-sync ./triage.sh

Accept label id (from hey label list) and, if unambiguous, label name.

Event should fire when:

• new mail arrives already labeled (contact auto-label, label address, or screener+autofile), or
• an existing thread gains that label (the interesting case for "I tagged this for the agent").

--events new should still mean new-to-this-watch in the same sense as box watch (unseen / unmuted / active since the watch began), plus the label-added case above so a late tag still wakes the helper.

JSON line can stay the current added/updated shape; including label {id,name} on the event would let --run-async scripts route without a second hey label view.

Workaround today

hey watch --box imbox --events new --run-sync then hey thread read / inspect labels and no-op. Works, but every Imbox add pays the script tax, and a label applied to an already-seen thread never fires.

Happy to bikeshed flags (--label vs --in-label) if you want it consistent with hey search --label.

[TomFaulkner]

Opened #488 with a v1 implementation.

v1 = new mail that already has the label; label-added-on-existing-thread only if the feed includes it (filtering uses folders already on the posting change payload — no extra polling).