cli: standard command surface + Mermaid export; full suite passing

[fruwe]

Build-order steps 15–16 (issue #3): the standard CLI and Mermaid export. This passes conformance/cli/01 and completes the definition of done — all 22 engine cases + conformance/cli/01.

What

• Mermaid export (§12) behind a pluggable exporter interface: static structure (composites as state S { … }, orthogonal regions separated by --, [*] --> initial, final --> [*], S --> T : event [guard], after(d)), plus current-state highlighting via classDef active / class … active when given a state_config.
• Standard CLI (§13) — validate / export / new / send / advance / env / state / list / snapshot / restore over a file-backed store (--store, default $HAREL_STORE or ./.harel), with the normative --json shapes and exit codes (0/2/3/4/5/1) and the virtual clock. State-changing commands run all instances to quiescence and persist atomically.
• A CLI harness runs each conformance/cli/* case against a fresh temp store (exit + structural-JSON assertions).

Definition of done — all met

• Engine passes the full conformance suite (01–22).
• Schema validation enforced on load.
• CI green on ubuntu-24.04 and macos-latest.
• Snapshot round-trip + ≥1 migration case passing.
• export produces valid Mermaid (static + current-state highlight).
• CLI matches the §13 surface and passes conformance/cli/.

Verification

• ruff check . ✓ · mypy src/harel ✓ · pytest -q → 129 passed (22 engine + 1 CLI + unit tests; 0 skips).

One follow-up for the repo owner (branch protection)

Per the issue's "Tooling / CI" section, when CI is green add a required_status_checks rule to protect_main with the two contexts test (ubuntu-24.04) and test (macos-latest). I've left branch-protection untouched (it's a governance change best made by the owner).