v26.05.23

docs: Readme — self-packaging feature bullet + dedicated section

Adds a self-packaging entry to the Features list (with anchor to the
new section) and a dedicated '📦 Self-packaging (single-binary deploy)'
section after the YAML/env-vars section.

The section covers:

• pack/info/unpack invocation
• runtime mechanism (bundle locator + EmbeddedArchiveFileProvider +
  embed:// DuckDB FS) and the macOS reserved-segment variant
• secret deny list at pack time (*.env, secrets/*, *.pem,
  *.key)
• reproducible builds via SOURCE_DATE_EPOCH
• 12-factor env-var precedence (FLAPI_CONFIG / FLAPI_LOG_LEVEL)
• macOS notes (FLAPI_RESERVED_BUNDLE_MIB, --macos-append legacy)

Cross-links to docs/CLI_REFERENCE.md §3 and
docs/spec/DESIGN_DECISIONS.md §9 for the upstream reference and
rationale.