Merge origin/main into EGB-283 (multi-recipient age encryption)

Reconcile the multi-recipient branch (cut from v0.6.1.0) with origin/main
at v0.7.4.0. The two feature lines are largely orthogonal; the one real
integration point is the external-blob encrypt path:

- EGB-712 added an additive-v2 dual-write loop (_external_blob_write_targets,
  writing v2 + any v1 twin). EGB-283 routes every encrypt site through
  RECIPIENT_ARGS for N-recipient encryption. Resolution keeps the dual-write
  loop but encrypts each target to the full recipient set
  (age "${RECIPIENT_ARGS[@]}" per write target), so dual-write and
  multi-recipient compose. cmd_push loads recipients before both external
  push sites; legacy single-key rekey keeps its fresh-keypair pubkey path.

Version: 0.6.2.0 + 0.7.4.0 -> 0.7.5.0. Docs (CLAUDE.md/README/CHANGELOG)
merged to carry both feature sets; subcommand list now includes
recipients/reencrypt and upgrade.

Tests: full `bats test/` green except 6 pre-existing host-environment
failures (4 chmod-600 restore assertions + 2 jq-PATH-shadow tests, all
macOS-authored), none touching merged code. recipients.bats 34/34 pass;
external/dual-write area passes except the same mode-600 host artifacts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

VERSION

@@ -1 +1 @@
-0.6.2.0
+0.7.5.0