fix(notify): default every notification category to Silent

[janicduplessis]

Description

The phone app and Stim Desktop alerted by default for the machine and control categories, with every other category defaulting to Silent. The server's own fallback delivery — used when a registered phone's levels do not cover a category — matched that split: loud for everything but started. The maintainer decided on 2026-09-28 that every category should start Silent instead, so a fresh phone or Mac never alerts until the person turns a category on.

Solution

DEFAULT_LEVELS (phone) and OversightCategory.defaultLevel (Desktop) now return Silent for every category. The shared oversight.ts/Oversight.swift notification builders (duplicated identically across apps/mobile/src/lib/oversight.ts, packages/server/src/oversight.ts and apps/desktop/Sources/StimKit/Oversight.swift, per the file's own "holds the same code" comment) built each OversightNotification with quiet: category === 'started', so the machine builder and the control conflict push in packages/server/src/push.ts also had their own hardcoded quiet: false; all of these now default to quiet: true. That quiet field only matters as the fallback in push.ts's levelOf when a phone's levels omits a category — a real phone always sends the full map from DEFAULT_LEVELS, so this only affects an older/partial registration.

Users' saved level choices are untouched: parsePrefs (phone) and NotificationSettings.level (Desktop) only fall back to the default for a category with no stored value.

Test plan

• pnpm test (root) and apps/mobile: full suites pass, including updated expectations in notifications.test.ts, oversight.test.ts and push.test.ts for the new default (e.g. a control conflict or a category missing from a registration's levels now delivers silently; two tests that were exercising the hourly-budget/loud-vs-silent behavior now pin an explicit alert level so they still exercise that path).
• swift test in apps/desktop: 281 tests across all three targets pass, including NotificationSettingsTests.defaultsEveryCategoryToSilentAndRunsEveryCategory and the OversightTests cross-check against the regenerated oversight-vectors.json fixture (only its quiet fields changed).
• Phone: built and installed on a Stim-owned iOS Simulator, paired to pnpm run mock-server, and enabled notifications in Settings — every category (Work started, Agent looks stuck, Agent repeats the same failure, Work finished or PR ready, Machine in trouble, Someone takes over your device) now shows Silent.
• Desktop: tried to confirm the same on a Mac mini test copy via mini-desktop.sh, but couldn't get a clean screenshot of the Settings window on the shared, heavily-loaded mini (other agents' windows and dialogs kept occluding it); relying on the Swift test coverage above instead, since AppPreferencesView.swift's picker binds directly to the now-tested category.defaultLevel.

Fixes #1813