fix(assistant): canonicalize rule file storage

[kaizhou-lab]

Summary

• percent-encode assistant IDs and locales used in rule/skill filenames so generated IDs such as bare:... do not create Windows alternate data streams
• read and migrate exact legacy rule paths while preserving deterministic non-empty locale fallback for assistant details and conversation snapshots
• remove unused assistant_definitions.rule_inline_content, source_version, and source_hash columns with migration 024

This addresses an independently verified Windows filename defect. Existing evidence does not prove that it caused Sentry ELECTRON-3EG, so this PR does not close that issue.

Verification

• cargo test -p aionui-assistant read_rule_user_
• cargo test -p aionui-app t1_3b_create_persists_available_locale_fallback_rule_in_assistant_snapshot
• cargo test -p aionui-db migration_024_removes_unused_fields_and_preserves_definition
• affected-crate clippy with -D warnings
• migration immutability check, workspace fmt, and workspace clippy passed through just push

Full workspace nextest was stopped before completion per request and is not reported as passing.