Add a What's new sheet after updating

[claude]

What & why

Before: after updating, nothing in Blurt says what changed. The only release notes are the auto-generated list of PR titles on the GitHub release.

After: the first launch after an update opens a sheet with the new version's highlights from CHANGELOG.md (up to three features, each with an SF Symbol, title and one sentence, then a Fixes list). "See all releases" links to GitHub Releases, and Return / Continue dismisses it. Help → What's New in Blurt reopens it for the running version, or the newest entry before it.

How: the rules live in the engine, as value types with tests:

• Changelog parses CHANGELOG.md (format documented in a comment at the top of the file). Changelog.whatsNewAtLaunch(current:lastSeen:isConfigured:hasRunBefore:) decides what to show and whether to record the version.
• LastSeenVersionStore stores the version on a new DefaultsKey.lastSeenVersion, so Reset Blurt clears it.
• The shell only loads the file, applies the decision in AppDelegate, and renders WhatsNewSheet. The sheet follows the APIKeyEditorSheet layout: 440 pt wide, 20 pt padding, glassButtonStyleCompat(prominent: true), which is .borderedProminent.

Bundling: a postCompileScripts phase in project.yml ("Bundle CHANGELOG.md") copies the repo-root file into the app's Resources. Its inputs and outputs are declared, so it only re-runs when the file changes. I didn't use a resource entry because the file sits outside the target's sources, and a resource entry would add a repo-root group whose name depends on the checkout directory.

About the .pbxproj change: I couldn't run xcodegen here, so I did not regenerate the project. I wrote the .pbxproj changes by hand to match what xcodegen produces: the new source file plus the new phase. The object IDs come from XcodeProj's own deterministic reference algorithm, which I checked against existing IDs in this project. The xcodegen drift check on CI is what confirms the result is byte-identical. If you'd rather, running xcodegen generate locally should produce no diff.

Fresh install vs. upgrade:

• Every launch of a build with this feature records a version, except an upgrade whose launch finds setup unfinished. That one waits for a configured launch.
• A fresh install's first run is never configured. It records its version without showing anything, so a later launch never reads it as an upgrade.
• A configured launch with nothing stored is an install from before this feature. To tell it apart from a reinstall or a UI-test launch, the decision checks whether an update check has ever completed (LastUpdateCheckStore). If one has, the sheet shows the current version's entry once, since the previous version isn't known. If not, it only records.
• A downgrade, or a relaunch of the same version, does nothing.

Because v0.1.57 is already out, anyone updating to the next build sees the 0.1.57 notes, unless that release adds its own entry first. AGENTS.md now says to add the entry before the bump.

How it was tested

This was written in a Linux sandbox with no Swift toolchain, so nothing Swift was built or run locally. CI on macos-26 is the authority on whether it builds and the tests pass.

Locally I ran:

• scripts/check-invariants.sh and its --self-test: pass.
• scripts/check.sh --portable: passes; swift-format, SwiftLint, actionlint, zizmor, shellcheck and shfmt were not installed, so those were skipped.
• prettier and markdownlint on CHANGELOG.md, AGENTS.md and project.yml: pass.

New engine suites: ChangelogTests (including a parse of the real CHANGELOG.md), WhatsNewLaunchTests (one test per launch case) and LastSeenVersionStoreTests. PersistedSettingsTests now covers the new key. The sheet itself hasn't been looked at on a Mac yet.

• scripts/check.sh passes (or CI will, if I'm not on a Mac)
• I read AGENTS.md and this doesn't reintroduce anything deliberately removed
• Docs updated if behavior changed

🤖 Generated with Claude Code

[alexkroman]

Implemented in #214