feat(fetchers): add StackOverflowFetcher for clean Q&A extraction (#64)

## What
Adds a `StackOverflowFetcher` for Stack Overflow and Stack Exchange
network question URLs, returning structured Q&A content via the Stack
Exchange API.

Closes #53

## Why
Coding agents frequently encounter Stack Overflow links. The
DefaultFetcher returns full pages with ads, sidebars, and noise. This
fetcher yields just the signal: question, answers, and votes.

## How
- Matches `stackoverflow.com/questions/{id}` and SE network sites
(serverfault, superuser, askubuntu, *.stackexchange.com)
- Fetches via Stack Exchange API v2.3 with `withbody_markdown` filter
- Returns: question title, body, score, views, tags, author, top N
answers sorted by votes
- Accepted answers marked with indicator
- Format field: `"stackoverflow_qa"`

## Risk
- Low
- Only adds a new fetcher; no changes to existing behavior

### Checklist
- [x] Unit tests passed
- [x] Clippy clean (`-D warnings`)
- [x] Docs build without warnings
- [x] Formatting applied

Author: chaliy

crates/fetchkit/src/fetchers/mod.rs

@@ -8,13 +8,15 @@ mod docs_site;
 mod github_code;
 mod github_issue;
 mod github_repo;
+mod stackoverflow;
 mod twitter;
 
 pub use default::DefaultFetcher;
 pub use docs_site::DocsSiteFetcher;
 pub use github_code::GitHubCodeFetcher;
 pub use github_issue::GitHubIssueFetcher;
 pub use github_repo::GitHubRepoFetcher;
+pub use stackoverflow::StackOverflowFetcher;
 pub use twitter::TwitterFetcher;
 
 use crate::client::FetchOptions;
@@ -121,8 +123,9 @@ impl FetcherRegistry {
     /// 2. GitHubIssueFetcher - handles GitHub issue/PR URLs
     /// 3. GitHubRepoFetcher - handles GitHub repository URLs
     /// 4. TwitterFetcher - handles Twitter/X tweet URLs
-    /// 5. DocsSiteFetcher - handles docs sites and llms.txt URLs
-    /// 6. DefaultFetcher - handles all remaining HTTP/HTTPS URLs
+    /// 5. StackOverflowFetcher - handles Stack Exchange Q&A URLs
+    /// 6. DocsSiteFetcher - handles docs sites and llms.txt URLs
+    /// 7. DefaultFetcher - handles all remaining HTTP/HTTPS URLs
     pub fn with_defaults() -> Self {
         let mut registry = Self::new();
         // Register specialized fetchers first (higher priority)
@@ -131,6 +134,7 @@ impl FetcherRegistry {
         registry.register(Box::new(GitHubIssueFetcher::new()));
         registry.register(Box::new(GitHubRepoFetcher::new()));
         registry.register(Box::new(TwitterFetcher::new()));
+        registry.register(Box::new(StackOverflowFetcher::new()));
         // DocsSiteFetcher for docs sites and llms.txt
         registry.register(Box::new(DocsSiteFetcher::new()));
         // Default fetcher last (catches all remaining URLs)
@@ -289,13 +293,14 @@ mod tests {
     #[test]
     fn test_registry_with_defaults() {
         let registry = FetcherRegistry::with_defaults();
-        assert_eq!(registry.fetchers.len(), 6);
+        assert_eq!(registry.fetchers.len(), 7);
         assert_eq!(registry.fetchers[0].name(), "github_code");
         assert_eq!(registry.fetchers[1].name(), "github_issue");
         assert_eq!(registry.fetchers[2].name(), "github_repo");
         assert_eq!(registry.fetchers[3].name(), "twitter_tweet");
-        assert_eq!(registry.fetchers[4].name(), "docs_site");
-        assert_eq!(registry.fetchers[5].name(), "default");
+        assert_eq!(registry.fetchers[4].name(), "stackoverflow");
+        assert_eq!(registry.fetchers[5].name(), "docs_site");
+        assert_eq!(registry.fetchers[6].name(), "default");
     }
 
     #[test]