feat: validate hook schema compatibility when shared across conventions

[rorybyrne]

Context

PR #66 code review identified that feature_tables hooks can be shared across conventions without schema-match validation. When two conventions register the same hook name, the feature table created for the first convention may have a schema incompatible with the second convention's hook output.

Current behavior

Hooks are shared across conventions by design — a single hook image can be referenced by multiple conventions. The feature_tables system creates one table per hook name (not per convention × hook). This works correctly when conventions use the same hook image with the same output schema.

Problem

There is no validation at convention registration time to ensure that when a second convention references an existing hook name, the feature schema matches the already-created table. If schemas differ, inserts will silently fail or produce incorrect data.

Proposed solution

When a convention is registered and references a hook name that already has a feature table:

1. Compare the new hook's feature_schema columns against the existing table schema
2. If incompatible (different column types, missing required columns), reject the convention registration with a clear error message
3. If compatible (subset or identical), allow it

Not in scope

• Renaming hooks per-convention (hooks are intentionally shared)
• Schema migration for existing feature tables