feat: name the task a comment belongs to so mentions reach its feed

[puemos]

Why

A comment on an artifact or canvas points at an id that lives in a run's JSON, not in a table the server can join against, so the backend had no way to work out which task the comment belonged to. Mentions on those comments reached people by email and the web inbox but never by this app's Activity page — the one surface where the comment is actually readable.

Changes

The task id now rides along in item_context (declared on commentContextSchema, which strips unknown keys, so it has to be part of the schema to survive a round-trip). useCreateComment takes the task id and merges it in at the single choke point all three call sites go through, so the optimistic row and the request agree.

It is client-supplied, so the backend checks it against the team before acting on it and re-checks visibility when the feed is read.

Stacked on feat/artifact-comments (#3970). Pairs with PostHog/posthog#75910, which consumes it — this is inert until that lands, since item_context is free-form JSON.

Testing

TaskCommentsList and ArtifactPreview suites pass (39 tests), typecheck and lint clean on @posthog/ui and @posthog/core. No manual testing — no running app in this environment. The behaviour this enables is only observable once the backend side is deployed.

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit a49bd9b.}

[puemos]

Folding this into #3970 — it belongs with the rest of the artifact-comments work, no reason for a separate PR.