chore(fdroid): prepare repo for F-Droid submission

[Xitee1]

Prepares the repo so F-Droid can build SleepTimer from source. F-Droid builds from source and needs three things this repo could not provide: a statically-parseable version, an unsigned release APK, and a changelog for every released versionCode.

Changes

• app/version.properties (new) — literal versionName/versionCode for F-Droid's checkupdates (UpdateCheckData). The Gradle build still derives the real values from the git tag via axion-release; this file is not read by the build. Bumped per release.
• app/build.gradle.kts — -PdisableSigning now emits an unsigned app-release-unsigned.apk for F-Droid to sign itself. The debug-signing fallback for keyless local/CI builds is preserved; release signing with SIGNING_KEYSTORE_PATH is unchanged.
• fastlane changelogs — add 101000 (v1.1.0) and 101010 (v1.1.1) in en-US and de-DE; move the misfiled Shizuku-startup-race note out of 100010 (v1.0.1) into 101000, where PR fix: timer runtime correctness, theming, and Shizuku UserService migration #39 actually landed it.
• .github/workflows/release.yml — new guard step fails the tag build if version.properties disagrees with the tag or a changelog is missing / over 500 chars.
• AGENTS.md (via CLAUDE.md symlink) — documents the per-release checklist: bump version.properties + hand-write both changelogs before tagging.
• README.md — correct compileSdk platform 35 → 36.
• docs/plans/2026-07-06-fdroid-submission.md — the fdroiddata recipe and submission steps for reference (the recipe itself lives in the fdroiddata fork, not this repo).

Verification

• ./gradlew :app:assembleRelease -PdisableSigning → app-release-unsigned.apk (no signature, no MANIFEST.MF) — what fdroidserver globs for.
• ./gradlew :app:assembleRelease → app-release.apk, debug-signed — keyless builds still work.
• ./gradlew lint → clean.

Follow-ups (not in this PR)

• After merge: tag v1.1.1 to publish the release the fdroiddata recipe points at.
• Run the reproducible-builds probe (apksigcopier vs the v1.1.1 GitHub APK) to decide whether F-Droid publishes your signature or its own — a one-way door at inclusion time. See the docs/plans note.
• Open the fdroiddata MR (needs a GitLab account).

🤖 Generated with Claude Code

[Xitee1]

Reproducibility verified (preliminary) + native-lib fix added

Ran a reproducibility probe against the existing v1.1.0 release before committing to the one-way-door signing decision: built the v1.1.0 tag from a clean clone (unsigned) and compared zip entries by CRC against the signed SleepTimer-v1.1.0.apk.

Result: 121 of 122 entries were already byte-identical — all DEX, resources, and resources.arsc. The only mismatch was libdatastore_shared_counter.so (DataStore's native lib, ×4 ABIs): GitHub Actions strips it host-dependently via the runner's NDK, while a plain build keeps the pristine AAR bytes (which are identical on every machine).

Fix (bc… in this PR): packaging { jniLibs { keepDebugSymbols += "**/*.so" } } so AGP doesn't strip native libs. Verified the four .so CRCs then equal the datastore-core AAR CRCs. With this in the v1.1.1 tag, GitHub's release APK is also unstripped → F-Droid's from-source build reproduces it → the app can ship under your signature (AllowedAPKSigningKeys 87e5fe65…) rather than F-Droid's key, so users cross-update between GitHub and F-Droid.

The recipe draft in docs/plans/ is updated to the reproducible-builds form. The definitive check is F-Droid CI on the submission MR; given only the now-fixed .so differed, it should pass.