feat(telemetry): instrument the three unmeasured steps of the PLG funnel

[viktormarinho]

Why

The PLG funnel dashboard maps 17 steps to events. Three had no event at all, so on the board a missing bar and a step nobody reached looked identical:

Step	Status before
12 · Board opened	no event
14 · Task marked done	no event — task_run_completed is the agent's run finishing, not a person
15 · Limit banner shown	no event — task_quota_blocked records the server-side block, not whether anyone saw the paywall

What

• task_board_opened — ref callback on the board root, not an effect (banned by plugins/ban-use-effect.ts). A dataset flag guards a re-invoked ref from double-counting, mirroring trackGateRef in reports/auth-gate.tsx.
• task_marked_done — fired inside the update mutation in useTaskBoardItemActions. That is the single choke point every status change routes through, so drag-drop, the bulk action bar and the task dialog are covered by one call site instead of ten scattered ones.
• task_limit_banner_shown — in onDelegateError, where the paywall dialog is actually opened. Carries kind so the three refusal reasons stay distinguishable.

All three carry organization_id, consistent with the other org-scoped events.

Follow-up (not in this PR)

The funnel still cannot be a native PostHog funnel end to end, because the Studio server events key on organization_id while the engine events key on domain. Two things surfaced while measuring this:

1. $process_person_profile: false silently drops group attachment. captureOrgEvent sets that flag when there is no user, and events sent that way arrive with $groups populated but $group_0 empty — so any unique_group math returns zero for them. task_run_enqueued and task_run_completed are affected; task_quota_claimed is affected only for its user-less half (67 of 84). Dashboard tiles were switched to count organization_id directly as a workaround.
2. Studio does not persist an org's store domain. COMMERCE_DISCOVERY_BIND takes siteUrl per call and the mapping lives in the engine's database. Persisting it and having captureOrgEvent attach domain would let one native funnel span the whole flow — worth its own PR, since it needs a migration and touches the shared telemetry path.

Verification

• bun run fmt, bun run check clean.
• bun run lint: 0 errors (18 pre-existing warnings, none in the touched files).

No tests: these are telemetry call sites in React, which TESTING.md's two-tier policy puts outside unit tests, and the e2e suite is not set up to observe a PostHog capture over the wire.

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Summary by cubic

Instruments three missing PLG funnel steps so funnel gaps are measurable. Previously these steps had no events; now we emit them without changing user flows.

• task_board_opened: Fires once per mount via a root ref guarded by a dataset flag. Sends organization_id.
• task_marked_done: Fires in the update mutation only when a person sets status to done. Sends organization_id and task_board_item_id. Not the agent’s task_run_completed.
• task_limit_banner_shown: Fires when the subscription paywall dialog opens. Sends organization_id and refusal kind.
• No API or UI changes; telemetry only.

^{Written for commit c2f5f1e. Summary will update on new commits.}