feat: improve base makefile usability

[jackchuma]

Summary

• Refactor Makefile to use an explicit ENV parameter (testnet-alpha, testnet-prod, mainnet) instead of a hardcoded ENV_NAME variable, so switching environments no longer requires editing the Makefile
• Add input validation (validate-env) with clear error messages when ENV is missing or invalid
• Add a help target documenting all available targets, environments, and overridable variables
• Read contract addresses (including LocalERC20) from per-environment deployment JSON files instead of hardcoding them
• Fix check-root and check-count to use the dynamic BASE_RPC and BRIDGE address instead of hardcoded values
• Remove the dev-deploy composite target

Test plan

• make help prints usage info
• make bridge-sol-to-solana ENV=testnet-alpha resolves correct RPC, deployment file, and contract addresses
• Running any target without ENV prints a clear error
• Running with an invalid ENV value prints a clear error
• make check-root ENV=mainnet uses mainnet RPC and bridge address

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1