#16830: fix: session lookup default

Fixes #16660 <!-- greptile_comment --> <h3>Greptile Summary</h3> Changed the default behavior for announce target resolution to prefer session lookup unless explicitly disabled. The condition was inverted from `!plugin?.meta?.preferSessionLookupForAnnounceTarget` to `plugin?.meta?.preferSessionLookupForAnnounceTarget === false`, making session lookup the new default for all channels that don't explicitly opt out. - This is a **breaking change** for channels like Discord that currently rely on fallback behavior (undefined treated as opt-out) - The existing test `sessions-announce-target.e2e.test.ts:64-72` expects Discord to skip the gateway call, but will now fail - Only WhatsApp currently sets `preferSessionLookupForAnnounceTarget: true` explicitly, so it's unaffected - All other channels (Discord, Telegram, Signal, etc.) will now perform session lookups by default The change needs either channel plugin metadata updates or test updates to align expectations. <h3>Confidence Score: 2/5</h3> - This PR introduces a breaking behavioral change that conflicts with existing test expectations - The logic change is intentional but breaks the existing test contract for Discord (and potentially other channels). The test explicitly expects Discord NOT to call `callGateway`, but the new logic will cause it to do so. This needs either: (1) Discord's plugin to explicitly set `preferSessionLookupForAnnounceTarget: false`, or (2) the test to be updated to reflect new expected behavior. Without knowing which channels should use session lookup vs fallback, this could cause runtime issues. - Pay close attention to `src/agents/tools/sessions-announce-target.ts` and the test file - the behavior change needs validation across all channel types _{Last reviewed commit: 2c8da99} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->