Fix nonexistent download script and README anchor in missing SEA binary error #353

Description

When the local SEA binary cannot be found, the SDK's recovery message points users to a script path and README anchor that do not exist:

uv run python scripts/download-binary.py
https://github.com/browserbase/stagehand-python#local-development

The repository script is actually scripts/download_binary.py (underscore), and the README has no Local development heading/anchor. CONTRIBUTING.md already contains the correct command.

Reproduction

From a source checkout without a downloaded binary:

from stagehand._custom.sea_binary import resolve_binary_path
resolve_binary_path()

The resulting FileNotFoundError recommends a command that fails because scripts/download-binary.py is absent.

Code references

• Incorrect command and dead anchor: src/stagehand/_custom/sea_binary.py:113-119
• Correct command: CONTRIBUTING.md:38
• Actual file: scripts/download_binary.py

Expected behavior

The exception should point to:

uv run python scripts/download_binary.py

and to an existing setup section (for example the relevant section in CONTRIBUTING.md).

Actual behavior

Users encountering the missing-binary error are sent to a nonexistent script and nonexistent README section.

Why this matters

This is the primary recovery path for local development and unsupported/missing wheel scenarios. The diagnostic currently turns one actionable setup error into two dead ends.

Prior-art check

I searched open/closed issues and PRs for both script spellings and the local-development error message. The stale hyphenated name originated in the canonical v3 import, but there is no open issue or pending fix for the current path.