fix(cli): preserve snapshot alpha and create shot dirs

[miguel-heygen]

What

• preserve transparency in full-frame and zoomed snapshot PNGs
• create missing parent directories for keyframes --shot output paths

Why

Transparent standalone overlays rendered correctly to VP9 alpha, but their snapshot review PNGs were forced to opaque RGB. The same workflow also failed with ENOENT when --shot targeted a file inside a directory that did not already exist. Both behaviors reproduce on CLI 0.7.52.

How

• pass Puppeteer omitBackground: true for snapshot screenshots and cropped snapshot captures
• recursively create the shot output parent before browser capture
• add regression coverage for both behaviors

Test plan

• bun run --filter @hyperframes/cli test -- --run src/commands/snapshot.test.ts src/commands/keyframes.test.ts src/capture/captureCompositionFrame.test.ts
• bun run --filter @hyperframes/cli typecheck
• bunx oxlint on the six changed files
• built the CLI and verified a transparent repro emits RGBA snapshot PNGs and writes a nested --shot path without pre-creating the directory