fix: drop default agent color pinning that collapsed TUI agent colors

PR #1093 pinned agent.config.color to theme tokens for every built-in
agent. On OpenCode v1 hosts this opts agents out of the TUI's round-robin
palette (the source of distinct per-agent colors) and resolves through
theme[color], where tokens collide: the mapping reuses accent (oracle,
council, councillors) and info (explorer, observer), and themes collapse
keys further (nord: primary == info; system/ANSI: primary == accent ==
info == cyan). Net effect: most agents render the same lake-blue color.

Stop setting colors by default. Colorless agents get the host TUI's
distinct theme-aware palette again; the validation, agents.<name>.color
and preset override path, and configured council -> councillor color
inheritance are preserved.

Fixes #1116