feat(stream-android): add design-matching skill

[aleksandar-apostolov]

Adds a design-matching skill for Android (Compose) — a repeatable method for reproducing a reference design (screenshot / Figma / "make it look like WhatsApp / Slack / Telegram / ") with the pre-built Compose components. This brings Android to parity with iOS on design-matching, but it is its own skill: built from the Android SDK's actual customization model and hardened empirically, not a translation of the iOS doc.

What's here

• skills/stream-android/design-matching.md — the skill: classify each screen (Step 0), decompose every region, route each difference to one of the two Android axes (theming token vs ChatComponentFactory slot), build, and verify region-by-region on the real navigation path (Step 5).
• SKILL.md — a styling-depth flag routes target-appearance requests to the skill (orthogonal to the A/B/C/D tracks).
• RULES.md — a "Matching a reference design" section (two axes, derived bubble-color tokens, composite-slot drop, the three most-missed regions, verify-or-it-doesn't-count).

Built for Android's customization model

Every mechanism is grounded in the Compose SDK, not carried over from another platform:

• Two axes, not three — Android has no Styles axis, so padding / corner-radius / bubble-shape / composer layout are structure (slot override), not a token.
• Bubble colors are derived internal vals — recolor via the brand ColorScale or a MessageBubble override, never a constructor param.
• ChatComponentFactory (~196 slots) is the structural surface; the doc routes each region to its slot and shows how to read the pinned version's source.
• Step 0 multi-screen classification (channel list vs message vs thread) and a "derive, do not clone" method so the skill generalizes to any reference instead of encoding specific apps.

How it was validated

Dogfooded one-shot from screenshots across three maximally-different archetypes, each built + verified on a live emulator against the reference, iterating region-by-region until it matched:

• WhatsApp — bubble messenger (in-bubble metadata, wallpaper, tail).
• Slack — flat workplace, channel-list + message screens, DM/channel split, multi-row composer.
• Telegram — bubble messenger with gradient wallpaper, per-user colored author names, in-bubble metadata, flat composer.

The composer, header, and channel-list-row guidance were hardened directly from visual grading of those runs (row-count / alignment / field-shape traps, the header trailing-avatar default, DM-vs-#, reactions placement, and "data-limited != verified").