fix(addie): stabilize upcoming event day count

[bokelley]

Summary

• match main's upcoming-event day calculation by rounding the day delta instead of flooring it
• add a 3.0.x changeset so the release PR consumes the source fix cleanly

Verification

• npx vitest run server/tests/unit/member-context.test.ts --pool=threads
• precommit: unit tests, dynamic import lint, typecheck
• pre-push: version sync, Mintlify broken-link check

[aao-release-bot]

LGTM. Clean backport of main's Math.round fix at server/src/addie/member-context.ts:1567 into the 3.0.x line — addresses the sub-millisecond drift that flipped "in 5 days" to "in 4 days" on the boundary.

Things I checked

• origin/main:server/src/addie/member-context.ts:1565 already uses Math.round with the explanatory comment — the source line in this backport matches.
• Sibling block at :1556 (last_published_at → "X days ago") correctly stays on Math.floor. "Elapsed time since" floors naturally; "time until" rounds. No inconsistency.
• Patch-level changeset is the right shape: prompt-string rendering only, no wire or schema surface touched. Filename is descriptive.
• Behavior change is bounded to the prompt string: events 13–24h out now render as "in 1 day" instead of "in 0 days," and events <12h in the past render as "in 0 days" instead of "in -1 days." Acceptable for a prompt fragment; no contract impact.

Follow-ups (non-blocking — file as issues)

• Backport drops the three-line explanatory comment that exists on main (origin/main:server/src/addie/member-context.ts:1557-1559). Carry it forward so a future reader doesn't wonder why this site rounds and the sibling floors, and revert the wrong one.
• No regression test landed with the backport. If main has a boundary-pinning test for this site, bring it across to 3.0.x so the line can't silently regress again.

Minor nits (non-blocking)

1. Changeset prose. .changeset/member-event-days-rounding.md leads with release mechanics ("Match the 3.1 line's..."). Something closer to "Round upcoming-event day count to nearest day so events exactly N days out don't intermittently render as N-1" describes the user-visible fix more directly. Non-blocking.

code-reviewer: sound-with-caveats (caveats are the dropped comment and missing test backport — both Follow-ups, not blockers).

LGTM. Follow-ups noted below.