Refactor command registration with shared helpers and metadata

[da-ba]

Motivation

• Centralize and formalize per-command metadata (icon + description) so the // command list is driven by a single source of truth.
• Reduce duplicated cache/load and fire-and-forget side-effect boilerplate across command implementations.
• Make it simpler and safer to add new commands that automatically appear in the command list.
• Improve test coverage for the registry and the new helper utilities.

Description

• Added CommandMetadata type, extended registerCommand(name, spec, metadata?) to optionally store metadata, and added getCommandMetadata for lookup.
• Introduced src/content/commands/command-helpers.ts with getOrLoadCommandCache (cache-or-load helper) and runNonBlocking (ignore non-critical async errors).
• Updated the // command list to read metadata via getCommandMetadata with a fallback and removed the duplicated hardcoded metadata map.
• Migrated /emoji and /mention to use the new helpers and updated many command registrations (giphy, emoji, font, mermaid, mention, now, kbd, link) to include icon + description metadata, plus docs and tests adjustments.

Testing

• Ran the unit test suite with bun run test, which succeeded (all tests passed).
• Ran the repo check with bun run check, which initially failed due to Prettier formatting, then fixed formatting with bunx prettier --write and re-ran bun run check, which succeeded.
• New/updated automated tests: src/content/commands/registry.test.ts and src/content/commands/command-helpers.test.ts were added/updated and executed as part of the test run.

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Codex Task

[github-actions]

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