feat(fctl-plugin): co-release the independent Ledger plugin

[flemzord]

Ledger's fctl commands need an independently installable executable now that fctl no longer owns the product implementation. This PR brings the existing product-owned plugin to release/v3.0 under misc/fctl-plugin, preserving the command/form contract, payload validation, page cursors, exact JSON integers, partial bulk results and business errors.

Following Auth #162, fctl-plugin-ledger uses only the public github.com/formancehq/fctl/pkg/pluginsdk boundary and transport.Serve. Offline --manifest and --version report the exact co-released service version and a separate plugin revision. GoReleaser packages Linux/macOS/Windows for amd64/arm64.

Tagged release CI preflights all six executable identities before upload, publishes raw OCI executable layers to ghcr.io/formancehq/fctl-plugin-ledger, and attaches fctl-plugin-catalogue.json generated from the native manifest, actual binary checksums and immutable OCI digests. Publication uses the job token, with isolated registry credentials. Nightly jobs do not perform catalogue publication.

Validation:

• bash scripts/agent-check and AI_REVIEW_BASE_SHA=8e9bb3ebc13f9235fa20e92fb45018a6f7e91e16 bash scripts/agent-check-pr: PASS. The selector ran full Nix/Just normalization, baseline and complete root-module race tests. No generated drift remains.
• Plugin race tests and vet/lint, including actual SDK subprocess HTTP brokering, payload/forms/pagination, partial errors and preflight/upload failure cases.
• Six real GoReleaser snapshot archives and checksums; real ORAS local layouts and catalogue, without hosted publication.
• Actual fctl executable (c07b2b5 reference) plus actual Ledger plugin against controlled local APIs: install/show, execute, exact integer request/result, partial bulk error, exact-version drift refusal before mutation, offline help and completion.
• Test recipes allocate a short temporary root and clean it on exit, fixing the macOS Unix socket path limit exposed by the canonical validator.
• Root and standalone GoReleaser configuration checks; validation-selector race tests; dependency inspection confirms no fctl core, Cobra or Ledger server imports.

First-publication follow-up: review/merge this Draft through normal gates, choose a service tag containing the changes, make the GHCR package Public after first upload, verify anonymous downloads/checksums, then separately promote the six release entries into formancehq/fctl-plugin-registry. The README includes commands and failure/retry behavior. Hosted GHCR permissions and first-release upload remain to be verified by that publication.

No fctl changes, tag creation, release trigger or merge performed.