test: add E2E lifecycle tests and Robinhood fork tests

[RunTimeAdmin]

Summary

Closes the main testing gaps identified in the readiness review before mainnet:

1. End-to-end path covering register → stake → attest → epoch → slash → zero-reputation
2. Foundry fork tests against live Robinhood testnet state

Changes

E2E Integration Tests (test/E2E.t.sol)

Comprehensive lifecycle tests covering:

• Full happy path: Register agent → deposit stake → propose/finalize reputation → initiate/execute slash → verify reputation zeroed
• Slash dispute: Committee initiates slash → operator disputes within window → agent reinstated
• Reputation challenge: Committee rejects bad oracle proposal during challenge window → original score preserved
• Unbonding enforcement: Cannot claim withdrawal before period elapses
• Slash sweeps queue: Slash confiscates funds even if operator tried to dodge by queuing withdrawal
• Multiple epochs: Score evolution over time with progressive updates
• Multiple agents: Independent lifecycle validation
• Terminal slash: Slashed agents cannot be reactivated

Fork Tests (test/Fork.t.sol)

Validation against live Robinhood testnet (46630) deployment:

• Verify deployed bytecode at addresses from deployments/46630.json
• Interface validation (ERC20, Identity, Reputation, Staking)
• Cross-contract role linkages (Staking has STAKING_CORE_ROLE on Identity/Reputation)
• Read path smoke tests against live state
• Simulated write paths using vm.prank without requiring mainnet keys
• Graceful skip when FOUNDRY_FORK!=1 so CI stays green without external RPC

CI Updates (.github/workflows/ci.yml)

• Added optional contracts-fork job that runs only when ROBINHOOD_RPC_URL secret is configured
• External contributors' PRs skip fork tests (main test suite still runs)
• Maintainers can enable fork validation by adding the secret

Documentation (README.md)

New "Testing" section documenting:

• Unit tests (default forge test)
• E2E integration tests (--match-contract E2EIntegrationTest)
• Fork tests with instructions for running locally with different RPC options

How to Run

Unit + E2E Tests (no external network required)

forge test                        # all tests
forge test --match-contract E2EIntegrationTest -vvv  # just E2E

Fork Tests (requires Robinhood testnet RPC)

# Option 1: foundry.toml alias (rate-limited public RPC)
FOUNDRY_FORK=1 forge test --match-contract ForkTest --fork-url robinhood_testnet -vvv

# Option 2: Alchemy (higher rate limits)
FOUNDRY_FORK=1 forge test --match-contract ForkTest --fork-url https://robinhood-testnet.g.alchemy.com/v2/YOUR_KEY -vvv

Test Results

All 192 tests pass:

• 9 E2E integration tests ✓
• 12 fork tests (skipped gracefully without FOUNDRY_FORK=1) ✓
• Existing unit test suite unchanged ✓

Ran 10 test suites: 192 tests passed, 0 failed, 0 skipped