Add 'linear' to the set of sources that carry a mapped GitHub login
(Slack, Linear, dashboard, schedule), so Linear-triggered runs resolve
the per-user OAuth token when the author_prs_as_user profile flag is
enabled. Previously only Slack and dashboard runs could open PRs as the
user; Linear runs always used the bot token.
The Linear webhook now resolves the GitHub login from the Linear email
via the same user-mapping store Slack uses, and passes it through the
run configurable and thread owner metadata.
Refs: 5003c953 (upstream #1683)
