feat: scope committed settings to each app

[janicduplessis]

Description

Monorepo apps currently share a repository-root .stim.json, so one app's runtime settings can affect another. Read committed runtime settings beside each app's package.json instead. Fixes #607.

Solution

CLI commands, doctor, and standalone Metro provider discovery use the app-local file without ancestor fallback. Relative SimSlim profiles, AVD fragments and committed providers resolve from the app. Explicit machine project/repository overrides retain their precedence; repository-wide worktree-copy rules stay at the main checkout root.

Monorepos must move runtime settings and adjust relative paths for each app, replacing the repository-scoped lookup added in #131. Single-app repositories keep their existing root file. The guide and website document the migration.

Test plan

In a real disposable Git monorepo and linked worktree, two apps resolved their own settings/providers/profiles, an unset app did not inherit root settings, and symlink identity plus root worktree exclusions remained intact. Clean worktree removal passed; this smoke checked configuration resolution, not native builds or devices.

[janicduplessis]

Fresh independent code review is clear after correcting two lifecycle guide lines from repo-level to app-level defaults. Settings/doctor/Metro reviewer suite passed189 tests. Full local checks passed; one real-Xcode metadata test intermittently exceeded its default5s timeout, tracked separately in #618; its focused rerun and the complete unit/e2e/runtime rerun passed without code or assertion changes. Real monorepo/worktree resolution smoke remains valid; this final commit changes only two guide words.