fix(webfetch): make format parameter optional with markdown default

[samcornor]

Summary

Fixes the webfetch tool failing when called without a format parameter.

Problem:

• The format parameter was required in the zod schema but not documented
• Calling webfetch without format produced validation error:

  Invalid option: expected one of "text"|"markdown"|"html"
  

Solution:

• Add .default("markdown") to the format parameter
• Update tool description to document format options
• Remove outdated "prompt" reference from description

Changes

• packages/opencode/src/tool/webfetch.ts: Add default value for format
• packages/opencode/src/tool/webfetch.txt: Update description

Testing

Verified the schema now accepts calls without explicit format parameter.