Support multi-tag filtering for todo list and update help example

[multiplex55]

Motivation

• Allow listing todos with multiple tag tokens (for example todo list @testing @ui).
• Keep backward compatibility with #-prefixed tags and existing ! negation semantics.
• Ensure tag filtering composes sensibly with existing fuzzy text matching and priority sorting.

Description

• Split the remainder of the todo list query into whitespace tokens, collecting tags prefixed with @ or # into requested_tags and other tokens into text_tokens.
• Apply tag filtering first by retaining only entries whose TodoEntry.tags contain all requested tags (case-insensitive).
• After tag filtering, apply existing fuzzy text filtering (with support for ! negation) against the joined text_tokens, and preserve priority sorting behavior.
• Update src/help_window.rs to include an example todo list @testing @ui.

Testing

• No automated tests were run.

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Codex Task