Prepare Workflow 2.28.0 product metadata and release notes - #1127

Moves the Unreleased entries under ## 2.28.0 - 2026-11-13 and sets product-train to 2.28.0, as for 2.27.0 (#1119). The section opens with the behavior-change line General asked for, and the #1117 entry is moved to the top of the section. No entry text changes; I checked this with a sorted-line diff against main, which shows only the heading and the four-line preamble as additions.

Contents:

• Record an outcome for activities, timers and select() losers still open when a run closes #1117 (Settle a run's open activities and timers when its own code closes it (#1117) #1121): behavior change. A run that completes, fails or continues as new cancels the activities and timers it never awaited, such as select() losers, in the closing transaction (cancellation_reason: run_closed). Late claims and outcomes are refused with run_closed, and replay ignores these cancellations. Service-mode cancellation_policy: abandon keeps running, with abandoned_activities on the terminal event and no outcome recorded.
• Record an outcome for activities, timers and select() losers still open when a run closes #1117 follow-up (Close a closed run's expired activity rows instead of re-sweeping them (#1117) #1124): the deadline sweep closes a closed run's expired open rows, without history, instead of re-selecting them forever.
• Cache workflow-code send candidates per identity during an execution #1014 (Load a send identity's candidates once per execution (#1014) #1125): a send identity's candidates are loaded once per execution, so N identical sends in a replay cost one query.

No migration. After merge, a GitHub release 2.28.0 on the merge commit triggers release verification. Then docs#526 (2.28 adoption plus the select() and continue-as-new guide lines) leaves draft.

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