Use sandbox thread key for company context live Slack

[fineas-bot]

Summary

• Let company_context resolve the active Slack channel from CENTAUR_THREAD_KEY when the CLI runs outside an in-process ToolContext.
• Keep live Slack gap-fill scoped to the current Slack channel while indexed company-context search can still retrieve public Slack context broadly.
• Add regression coverage for ToolContext precedence, env fallback, DM rejection, and live Slack search using the sandbox thread key.

Verification

• uv run --with pytest --with asyncpg --with slack-sdk --with rich pytest tools/productivity/company_context/tests/test_client.py (24 passed)
• Remote head commit cd80092e199d426f22c3c983c86f92486c749cad is GitHub verified (valid).