remote: support explicit local override for PLANNOTATOR_REMOTE

[foxytanuki]

Closes #480

Summary

PLANNOTATOR_REMOTE currently only gives explicit meaning to the remote side, so false / 0 still fall through to SSH auto-detection.

This PR aligns the runtime and docs around explicit tri-state semantics:

• 1 / true forces remote mode
• 0 / false forces local mode
• unset keeps the existing SSH-based auto-detection path

It also updates both Bun and Pi runtimes, adds regression coverage, and refreshes the related docs/readmes to match the actual behavior.

Notes

• Explicit local override now bypasses SSH_TTY / SSH_CONNECTION
• Unset PLANNOTATOR_REMOTE still preserves the legacy SSH auto-detect behavior
• Remote browser handling docs were updated to reflect that behavior depends on the configured browser handler and environment

Testing

• bun test packages/server/remote.test.ts packages/server/browser.test.ts apps/pi-extension/server/network.test.ts

[backnotprop]

Review from Plannotator