feat: Implement at-mention detection for Bluesky posts

[Copilot]

This PR implements at-mention detection for BlueSky posts, allowing mentions like @username to be properly converted to clickable links in BlueSky posts.

Changes Made

Core Implementation

• Added BLUESKY_MENTION_FACET constant for the BlueSky mention facet type (app.bsky.richtext.facet#mention)
• Created BlueSkyMentionFacetFeature class to represent mention facets in the BlueSky API format
• Added MentionDetails typedef for type safety
• Implemented detectMentions() function that uses a robust regex pattern to detect valid mentions
• Updated detectFacets() to include mention detection alongside existing URL and hashtag detection

Mention Detection Features

• Detects various mention formats: @username, @user.name, @user-name
• Handles mentions in different contexts:
  • Standalone: @alice
  • In parentheses: (@username)
  • Mixed with other content: Check @user for #javascript tips!
• Validates mention format (rejects @123, @., @@, etc.)
• Properly calculates byte offsets for BlueSky API compatibility

Testing

• Added comprehensive test suite with 29 new test cases covering:
  • Valid mention detection in various positions
  • Multiple mentions in the same text
  • Mentions combined with URLs and hashtags
  • Invalid mention rejection
  • Proper byte location calculation

Example Usage

import { detectFacets } from './src/util/bluesky-facets.js';

const text = "Hello @alice and @bob, check out https://example.com!";
const facets = detectFacets(text);
// Now includes mention facets for @alice and @bob along with URL facet

Future Enhancement

The current implementation stores handles in the did field as noted in the issue. A future enhancement will resolve handles to actual DIDs via the BlueSky API and validate mention existence before posting.

Testing Results

• ✅ All 267 tests passing
• ✅ ESLint passes with no issues
• ✅ Manual verification confirms mention detection works correctly
• ✅ Existing functionality unchanged

Fixes #103.

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[nzakas]

This correctly finds at-mentions, but we really do need to use the actual did when we start the request.

[Copilot]

You're absolutely right. The current implementation correctly detects mentions and stores handles in the did field as a placeholder. The next step will be to resolve these handles to actual DIDs via the BlueSky API before making the post request. I've noted this in the class documentation for future enhancement.

[nzakas]

Can you make that change now?

[Copilot]

I've implemented DID resolution for BlueSky mentions. The changes include:

• Added resolveHandle() function to resolve handles to DIDs via the BlueSky API
• Updated the posting flow to resolve mention handles before creating posts
• Added comprehensive tests covering both successful resolution and graceful failure handling
• Mentions that fail to resolve are silently skipped to prevent post failures

All 269 tests are passing. Commit: f681245