feat(instagram): add media download command

[fii6]

Summary

• add an instagram dl command to download media from /p/, /reel/, and /tv/ links using the active browser session
• fetch Instagram media metadata inside the logged-in browser context, then download the resolved image/video URLs locally
• default downloads to /sdcard/Pictures/Instagram, support --path to override the destination
• keep command output minimal: no result table, no progress spam, only 📁 saved: ... after completion
• place the new command after saved in Instagram help output and keep the command description in English

Validation

• npm run build
• vitest run src/clis/instagram/download.test.ts --pool=threads --maxWorkers=1
• real download test via CDP-backed browser session:
  • opencli instagram dl "https://www.instagram.com/p/DWUR_azCWbN/" --path ~/tmp/instagram-dl-alias-test

[jackwener]

Merged after review. I specifically looked at the design intent here: the new Instagram media command should fit the existing OpenCLI command model instead of introducing a one-off UX path. I renamed it to the repository-wide download convention, removed the global ordering change, switched the default output directory to a cross-platform Downloads path, and validated with targeted Vitest, commanderAdapter/build-manifest coverage, typecheck, and build before merge. Thanks for the contribution.