feat: name the tool bridge and runtime after the app hosting OpenCode

Claude was always told it runs in OpenChamber and saw its tools as
mcp__opencode__*, even for people using plain OpenCode.

OpenChamber starts its managed OpenCode with OPENCHAMBER_RUNTIME set. When
it is present, the bridge is named openchamber and the runtime note and
utility prompt say OpenChamber; otherwise both say OpenCode. Calls already
in a session under either name still map back to OpenCode's tools.

Tested with host-regression for both hosts. The suite now runs as plain
OpenCode whatever terminal starts it.