* fix(opencode): trust discovered modes instead of always injecting defaults
When OpenCode reports its available agents, respect that list exactly
rather than seeding from DEFAULT_MODES first. This means users who have
intentionally disabled modes (e.g. Plan) in their OpenCode config will
no longer see those modes surfaced in Paseo.
DEFAULT_MODES is still used as a fallback when discovery returns nothing
(e.g. OpenCode unreachable or no agents configured).
* test(opencode): update mode discovery expectations
* test(opencode): expect discovered custom modes only
