What

Adds suspend fun unarchive(conversationId: String) to ConversationRepository and implements it in FakeConversationRepository as the inverse of archive(). Clears the archived flag so the conversation reappears in the Discussions (or Channels) stream and leaves Archived.

Issue

Closes #96

Architecture compliance

Follows the spec at docs/specs/architecture/96-unarchive-primitive.md exactly:

• Interface method placed immediately below archive() (line 32) — inverse pairing visible at a glance.
• Fake impl mirrors archive() structure (atomic state.update, throw unknown(id) for unknown IDs, copy(archived = false)).
• Idempotent on already-unarchived conversations: copy(archived = false) produces an equal Conversation value; StateFlow deduplicates equal emissions. Matches the established archive() convention.
• No domain-model change (Conversation.archived already exists from feat(data): retain conversations on archive() + expose archived filter + seed #93).

Testing

Three new unit tests in FakeConversationRepositoryTest, mirroring the existing archive_* cluster:

• unarchive_movesConversation_from_Archived_to_Discussions_andRetainsInStore — the AC test, using the existing seed-discussion-archived seed.
• unarchive_onUnknownId_throws — IllegalArgumentException on unknown ID.
• unarchive_isIdempotent — two consecutive calls leave the conversation in Discussions exactly once.

TDD: RED first (compile failure on unresolved unarchive), then GREEN after adding interface + impl.

Test fakes in ChannelListViewModelTest and DiscussionListViewModelTest got matching TODO("not used") stubs to satisfy the new interface member — same pattern used for archive() in those files.

Build status

• ./gradlew :app:testDebugUnitTest — pass
• ./gradlew :app:lint — pass (no new findings)
• ./gradlew :app:assembleDebug — pass
• ./gradlew :app:connectedAndroidTest — not run (no device/emulator connected in dispatcher env); change is data-layer only, no UI surface.

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