feat(data): add Conversation, Session, Message data classes (#2)

[ilmoniemi]

What

Creates the de.pyryco.mobile.data.model package with three foundational data classes and one enum:

• Conversation — id, optional name, cwd, current session, session history, isPromoted flag, lastUsedAt timestamp
• Session — id, conversationId, claudeSessionUuid, started/ended timestamps
• Message — id, sessionId, role, content, timestamp, isStreaming
• Role { User, Assistant, Tool } co-located in Message.kt

Wires kotlinx-datetime 0.6.2 via the version catalog (libs.kotlinx.datetime) so timestamps stay portable for a possible Compose Multiplatform walk-back — java.time.Instant would have locked the data layer to JVM/Android.

No ConversationRepository yet (next ticket), no persistence, no serialization annotations.

Issue

Closes #2

Testing

• New unit-test files under app/src/test/java/de/pyryco/mobile/data/model/ — one per data class.
• Each data class test asserts equals, hashCode, copy(...) overrides one field while preserving the rest, and copy() returns a new instance.
• MessageTest additionally asserts Role.entries is exactly [User, Assistant, Tool] in declaration order.
• ./gradlew test — passes.
• ./gradlew lint — passes.
• ./gradlew assembleDebug — passes.
• ./gradlew connectedAndroidTest — not run; no device connected and this ticket has no instrumented tests.

Architecture compliance

• File layout follows the spec: one class per file, Role co-located in Message.kt (tightly bound, three-line enum, no independent consumers).
• Field names, types, and nullability copied verbatim from AC1.
• kotlinx.datetime.Instant used throughout the data layer (CLAUDE.md "Don't" — keep data/ portable). No java.time.Instant import anywhere.
• Dependency declared in gradle/libs.versions.toml and pulled into app/build.gradle.kts via libs.kotlinx.datetime. No raw coordinate in the build file.
• No annotations on the data classes (Out of Scope: wire-protocol serialization deferred).
• No require(...) / init { } blocks — scope is the schema only.

🤖 Generated with Claude Code

[ilmoniemi]

Code Review: #2

Decision: PASS

Findings

No MUST FIX, SHOULD FIX, or NIT findings.

Verified against the spec at docs/specs/architecture/2-conversation-session-message-data-classes.md:

• Conversation, Session, Message, Role declared in de.pyryco.mobile.data.model with field names/types/nullability matching the AC1 spec block verbatim
• kotlinx.datetime.Instant used (CLAUDE.md "Don't" — data layer must stay portable for Compose Multiplatform walk-back); no java.time.Instant leak
• No @Serializable / @JsonClass annotations (Out of Scope respected)
• No init { } / require(...) blocks (no AC mandated validation; correctly avoided scope creep)
• kotlinx-datetime 0.6.2 wired through gradle/libs.versions.toml only — no raw coordinate in app/build.gradle.kts, correct catalog accessor libs.kotlinx.datetime
• File layout matches spec: Role co-located in Message.kt, otherwise one-class-per-file
• Tests use JUnit 4 idiom matching ExampleUnitTest.kt; three test methods per data class plus role_has_exactly_user_assistant_tool in MessageTest.kt
• Role.entries.toList() used (spec explicitly permits as the Kotlin 1.9+ replacement for values())
• Instant.fromEpochSeconds(0) used for deterministic fixtures; no Clock.System.now()

Local verification: ./gradlew test and ./gradlew assembleDebug both BUILD SUCCESSFUL on feature/2.

Summary

Textbook spec-following implementation of foundational data types. No consumers yet, so blast-radius is nil. Ready to merge; the next ticket (ConversationRepository interface + FakeConversationRepository) can build on this schema unchanged.