feat: remove shared contract error collisions

[OkeyAmy]

Closes #41

Motivation

• The workspace defined two separate #[contracterror] enums (common::ErrorCode and campaign::Error) whose discriminants overlapped, risking mis-attributed on-chain error codes when both crates are present in a downstream dependency graph.
• To eliminate the collision without renumbering stable on-chain discriminants, the change drops the shared ErrorCode from common and makes the campaign crate the canonical owner of campaign error discriminants.

Description

• Remove the #[contracterror] enum ErrorCode from common/src/lib.rs and stop importing contracterror there so common becomes a pure shared data-types crate.
• Update common crate metadata (common/Cargo.toml) and crate docs to document that it intentionally does not publish a #[contracterror] enum.
• Clarify campaign/src/types.rs docs to state that campaign::Error is the canonical campaign error enum and cannot collide with a shared error space.
• Add a focused unit test (campaign::types::error_code_tests::campaign_error_discriminants_are_unique_without_common_error_space) that asserts campaign error discriminants are internally unique.
• Add deployment notes to docs/deployment.md explaining the decision and that no migration sequencing is required because no campaign discriminants were renumbered.

[GBOYEE]

I'd like to work on this.

Approach:

• I'll analyze the codebase and implement a clean solution
• Verify with existing tests + add new ones if needed

Estimated effort: ~1-2 hours. PR incoming shortly.