feat(devex): add a fincore sandbox helper CLI

[tiana-code]

E-09 DevEx #302 (F-09.1) - the first E-09 slice: a sandbox helper CLI.

What

• scripts/fincore (POSIX bash, executable): init (scaffold a local .env with the documented non-secret dev defaults, clobber-guarded), status (curl the readiness of ledger:8080 + payments:8081 + web:8082, UP/DOWN, non-zero if any down), run-demo (exec scripts/demo.sh with pass-through args), help / unknown -> usage. set -euo pipefail, all variables quoted, no eval, fixed case dispatch.
• .github/workflows/shell.yml - a new shellcheck gate over scripts/fincore + scripts/*.sh.

Source-fidelity

• status covers exactly the services in docker-compose.yml (ledger/payments/web); compliance and decision are not in compose, so they are not invented.
• init defaults match the compose defaults (POSTGRES_PASSWORD=ledger, GRAFANA_ADMIN_PASSWORD=admin); .env is gitignored, nothing secret is committed.
• The CLI lives under scripts/ (alongside demo.sh) because bin/ is gitignored (.gitignore:22) - corrected mid-slice from the issue's bin/ wording.

Gate chain

• code-reviewer: APPROVED, no must-fix (shellcheck-clean by reading - all quoted, safe ROOT_DIR idiom, correct exit codes, ports match compose).
• security-auditor (opus): PASS - no shell injection (set -euo pipefail + quoting + no eval + fixed dispatch), no SSRF (hardcoded localhost), non-secret dev defaults, CI has no github.event injection and contents:read, no deploy/publish.
• evaluator: PASS (0.875).
  shellcheck is not installed locally; the new shell.yml CI job is the lint gate. Local smoke: bash -n clean, help/no-arg/unknown verified, exec bit 100755.

Closes #302