feat: write devnet manifest to shared data directory + Sepolia example

[Nic-dorman]

Summary

Move the devnet manifest from /tmp (and GUI-specific paths) to the shared ant data directory (ant_core::config::data_dir()). This lets any consumer — ant-gui, ant-cli, ant-tui — discover devnet/testnet mode by checking the same well-known location where daemon.port already lives.

Replaces #29 which was closed because the examples wrote directly to ant-gui's config directory, coupling ant-core to a specific GUI app.

Changes

• start-local-devnet (updated): 25 nodes (was 5), writes manifest to shared data dir, 8MB thread stack for Windows, cleanup on Ctrl+C
• start-devnet-sepolia (new): 25 nodes with ArbitrumSepoliaTest EVM, uses existing deployed Sepolia contracts, no wallet key embedded (user connects their own funded wallet)

Manifest location

Platform	Path
Windows	%APPDATA%\ant\devnet-manifest.json
macOS	~/Library/Application Support/ant/devnet-manifest.json
Linux	~/.local/share/ant/devnet-manifest.json

This is the same directory where daemon.port and node_registry.json already live.

Difference between the two examples

	Local Anvil	Sepolia
EVM	Embedded Anvil blockchain (localhost)	ArbitrumSepoliaTest (public RPC + deployed contracts)
Wallet key	Auto-funded by Anvil, included in manifest	Empty — user connects their own funded wallet
Use case	Local development, E2E tests	Testing against real testnet, pre-release validation

Impact on other tools

• ant-cli: No changes needed. Already has --devnet-manifest <path> flag. Could optionally auto-discover from the shared dir in the future.
• ant-core SDK: No changes. LocalDevnet and Devnet expose .write_manifest(path) — the caller decides where to write.
• ant-gui: Companion PR at feat: read devnet manifest from shared data directory ant-ui#9 reads from this shared path.

Future direction

This is the simplest approach ("Approach A"). Longer-term, the daemon itself should be network-aware — e.g. ant node daemon start --network sepolia — and expose the network config via its REST API (GET /api/v1/network). See discussion in ant-ui#9.

Test plan

• cargo run --release --example start-local-devnet starts 25 nodes + Anvil, manifest written to shared data dir
• cargo run --release --example start-devnet-sepolia starts 25 nodes on Sepolia, manifest written
• Manifest cleaned up on Ctrl+C
• ant-gui detects manifest from shared data dir (with ant-ui#9)

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