Ignore build artifacts and tool directories in .flowconfig

[captbaritone]

Summary

• Add .claude/ to Flow ignore list (Claude Code worktrees are full repo copies)
• Add website/build/ and website/.docusaurus/ to Flow ignore list (docs build artifacts)

These directories were being parsed by Flow unnecessarily, inflating the file count from ~2.5k actual project files to ~36k total.

Source	Files	Should be parsed?
.claude/worktrees/	14,246	No
website/build/ + website/.docusaurus/	2,851	No
Root node_modules/	16,629	Yes (haste resolution for fbjs, standard resolution for graphql, etc.)
Actual project files	~2,500	Yes

Test plan

Before this change:

$ ./node_modules/.bin/flow ls 2>/dev/null | grep -v '^Skipping' | wc -l
   36296

After:

$ ./node_modules/.bin/flow ls 2>/dev/null | grep -v '^Skipping' | wc -l
   19196

Breakdown by directory (after):

$ ./node_modules/.bin/flow ls 2>/dev/null | grep -v '^Skipping' \
    | sed "s|^$(pwd)/||" | cut -d'/' -f1 | sort | uniq -c | sort -rn
16629 node_modules
 2269 packages
  150 website
    9 scripts
    8 vscode-extension
    6 flow-typed
    ...

No .claude/, website/build/, or website/.docusaurus/ directories in the output.

Verify no new Flow errors introduced:

$ ./node_modules/.bin/flow status 2>&1 | tail -3
Found 2 errors

(The 2 errors are pre-existing cannot-resolve-name for performance — unrelated to this change.)