feat(data): add unarchive() primitive to ConversationRepository + Fake impl

[ilmoniemi]

User Story

As an app developer wiring the Archived Discussions UI restore action, I want the repository to expose an unarchive(conversationId) primitive that clears the archived flag, so the UI layer can restore an archived discussion and have it reappear in the active Discussions list.

Context

The Archived Discussions view (split from #77) needs a restore action. The foundation slice #93 introduced the archived: Boolean flag on Conversation, retains conversations on archive(), and exposes ConversationFilter.Archived for observing the archived stream. This ticket adds the trivial inverse: a primitive that clears the flag.

It is its own slice rather than bundled into #93 because pairing the inverse with the foundation would push the combined ticket over the AC limit; splitting also let the foundation land independently so the UI restore work (#94) can start as soon as this primitive lands.

Acceptance Criteria

• ConversationRepository interface exposes suspend fun unarchive(conversationId: String), mirroring the existing archive operation
• FakeConversationRepository.unarchive(conversationId) clears the archived flag on the named conversation (inverse of the flag-based archive semantics)
• Unit test: starting from a seeded archived discussion, unarchive(id) removes it from the ConversationFilter.Archived stream and adds it to the ConversationFilter.Discussions (active) stream

Technical Notes

• Stay consistent with the existing interface: raw String for conversationId, no ConversationId value type.
• For unknown IDs: mirror archive()'s behavior, which throws IllegalArgumentException("Unknown conversation: <id>") via the existing unknown() helper in FakeConversationRepository.
• Calling unarchive on a conversation that isn't currently archived is a no-op-equivalent (the flag is already false); the architect picks whether this is a silent success or an explicit precondition check.

Size Estimate

XS

Depends on

#93 (landed) — introduced the archived flag and the archived-observable stream. Linked via GitHub blocked-by.

Split from

#77

[ilmoniemi]

🤖 Product Owner — creates structured issues

po agent has completed work on this ticket.

Agent output (click to expand)

Refinement complete. Tightened the body to reflect that foundation #93 has merged (Context, "Depends on" instead of "Blocked by"), and clarified the unknown-ID / already-unarchived contract by referencing the actual `unknown()` helper now in `main` so the architect picks the precondition shape with full information.

Sizing kept at XS (interface line + ~6-line fake impl + ~15-line test); ticket is data-layer only so no Figma reference is required, and it touches no auth / token / network / crypto / untrusted-input surface, so `security-sensitive` does not apply.

Ready for human review. Move to the next column when approved.

[ilmoniemi]

🤖 System Architect — defines Go interfaces, data flows, concurrency patterns

architect agent has completed work on this ticket.

Agent output (click to expand)

Spec committed. Summary:

- **Sized XS, no split.** ~7 lines of production code (1 interface line + ~6-line method body mirroring `archive()`), 3 files touched (interface, fake impl, test), 3 AC. No edit fan-out (net-new method, zero call sites to update). No branch overlap on the touched files.
- **Design:** `unarchive(conversationId: String)` on `ConversationRepository`, exact inverse of `archive()`. Idempotent silent success on already-unarchived (matches the established `archive_isIdempotent` convention and the StateFlow's equal-value dedup); unknown ID throws `IllegalArgumentException` via the existing `unknown()` helper.
- **Tests:** Three cases mirroring the `archive_*` cluster — the AC round-trip using the existing `seed-discussion-archived` seed, unknown-ID throw, and idempotence.
- **No security-review pass** (no `security-sensitive` label) and **no Design source section** (no UI in this slice — pure data-layer primitive).

Ready for human review. Move to the next column when approved.