Promote prompt queueing (/queue + QueuedPromptsV2) to Preview

[harryalbert]

Description

Promotes the prompt queueing feature from Dogfood to Preview by moving both of its runtime flags into PREVIEW_FLAGS:

• FeatureFlag::QueueSlashCommand — the /queue slash command, auto-queue toggle, and the queued-prompts panel (regular Agent Mode).
• FeatureFlag::QueuedPromptsV2 — extends the same panel to Cloud Mode setup/follow-up draining and to /compact-and / /fork-and-compact.

Both flags previously lived in DOGFOOD_FLAGS. Because PREVIEW_FLAGS are automatically included in dogfood (WarpDev) and local builds (see app/src/bin/dev.rs and app/src/bin/local.rs), dev/local behavior is unchanged; the only effective change is that Preview (Friends of Warp) builds now enable prompt queueing. Stable is intentionally untouched — these flags are not in app/Cargo.toml's default set, so nothing ships to Stable yet.

The queueing code is always compiled in and gated at runtime via FeatureFlag::*.is_enabled() (per specs/APP-4041, specs/REMOTE-1543, specs/APP-4562, specs/APP-4594), so adding the flags to PREVIEW_FLAGS (applied at startup via ChannelState::additional_features()) is sufficient to enable the feature in Preview.

PendingUserQueryIndicator is intentionally left as-is: per specs/REMOTE-1543/PRODUCT.md, it is compatibility infrastructure for the legacy pending-user-query placeholders, not a rollout switch for the queue panel.

A single Preview-exclusive changelog description is added on QueueSlashCommand so the feature shows up once in the Preview in-app changelog (app/src/changelog_model.rs).

Linked Issue

• The linked issue is labeled ready-to-spec or ready-to-implement.

No single GitHub issue; this rollout covers the prompt queueing specs APP-4041, REMOTE-1543, APP-4562, and APP-4594.

Testing

• I have manually tested my changes locally with ./script/run

This is a feature-flag rollout change (no logic change). Validated with:

• cargo fmt -p warp_features (clean)
• cargo clippy -p warp_features --all-targets --all-features --tests -- -D warnings (clean)

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE

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Conversation: https://staging.warp.dev/conversation/9f00be34-baba-4543-94f4-79f4915048e2
Run: https://oz.staging.warp.dev/runs/019e8907-7c15-7554-86f5-79544f94774b

This PR was generated with Oz.

[harryalbert]

Superseded by #12081, which releases prompt queueing straight to prod (Stable) by enabling the queue_slash_command and queued_prompts_v2 cargo features by default, instead of the Preview-only rollout in this PR. Closing in favor of that one.