chore(registry): SX-3 review follow-ups (scanner encodings, scan memory, deploy guide env flags) #61

Follow-ups from the PR #59 round-2 review (non-blocking while SKILLX_RELEASES_API is off):

• Scanner false positives (r2-m1): UTF-16 .ps1 scripts (with a BOM), legacy-codepage .bat/.cmd/.csv files, binaries under scripts/, and LICENSE.txt may be quarantined. Decode UTF-16 when it has a BOM, and tune the injection scan for license and plain-text files.
• Scan memory (r2-m2): each file is built as a full string, plus a join and a decode. A 32 MiB tree can exceed the 128 MB isolate. Stream the scan per file, or cap the text size scanned.
• Budget guard: move it before the scan and the signing.
• Settings UI: label scoped keys.
• Deployment guide: it uses --env production / --environment production throughout, but wrangler.jsonc declares no environments. The production worker is skillx-web, so drop those flags across the guide and the README.

Refs #54, #59