feat(settings): add configurable app identity headers

[hanxuanliang]

Summary

Add configurable app identity settings so forks can override the default Bub attribution headers without patching source code.

Previously, the runtime hardcoded HTTP-Referer: https://bub.build/ and X-Title: Bub in agent.py. That meant forks and white-label builds had to carry a source patch just to update branding.

What Changed

• add BUB_APP_NAME and BUB_APP_URL to AgentSettings with the existing Bub defaults
• build extra_headers from settings instead of a hardcoded constant in agent.py
• allow empty or null config values to omit either header entirely
• document the new settings in env.example and README.md
• add regression coverage for default values, overrides, disabled values, and runtime header injection

Why

This keeps the public config provider-neutral while preserving the previous default behavior:

• default installs still send Bub identity headers
• forks can override branding in .env without carrying a source patch
• the runtime continues to pass app identity metadata through the existing extra_headers path

Tests

Added regression coverage for:

• default BUB_APP_NAME and BUB_APP_URL values
• .env overrides for app identity
• disabling headers with empty or null config
• runtime propagation of configured headers, including model override paths

Verification

• uv run ruff check .
• uv run mypy src
• uv run pytest -q