feat: allow init without explicit source arguments

[dapi]

Problem

memory-bank-cli update works without arguments, but init currently requires --source, --template-version, and --source-ref. This makes the first setup flow less convenient and inconsistent with updates.

Current behavior:

$ memory-bank-cli init
memory-bank-cli init: --source, --template-version, and --source-ref are required

Expected behavior

memory-bank-cli init should work without arguments, resolving its source, template version, and source ref through the same defaults/configuration path used by memory-bank-cli update.

Explicit flags must continue to override the resolved defaults.

Acceptance criteria

• memory-bank-cli init succeeds with no flags when the standard default/configuration source is available.
• Resolution semantics match memory-bank-cli update.
• --source, --template-version, and --source-ref remain supported overrides.
• CLI tests cover the no-argument init path and invalid/missing resolved configuration.

[dapi]

GH-41 implementation is on branch codex/gh-41 at 2d8d167. Verification passed: go test ./... . Pull-request creation is currently blocked by GitHub API and GraphQL 500 responses; no PR was created.

[dapi]

PR opened: #44

Verification performed:

• GOCACHE=/private/tmp/memory-bank-cli-gh-41-gocache go test -count=1 -race ./...
• GOCACHE=/private/tmp/memory-bank-cli-gh-41-gocache go vet ./...
• git diff --check