T1 — unit tests

Commit 1e92d8b (the implementation; later commits change docs only).

uv run pytest tests/unit -q

571 passed in 8.11s

New tests for issue-34: tests/unit/agents/test_roster.py (roster builder), tests/unit/core/test_context.py (agents section), tests/unit/core/test_spawn_agents.py (unknown, empty, known and null agent; registry read at call time) and tests/unit/prompts/test_prompt.py (## agents content, extends: agents).

After R5 and R6 (PR review)

tests/unit/test_config_card.py, 9 tests: an absent [card] equals the default description; a set section reaches the built card; an unset key keeps its default; five refusals (empty name, blank description, unknown key, duplicate skill id, empty skill id); the example's two configurations load and pair up. Red first: collection failed on ImportError: cannot import name 'CardConfig'. Then uv run pytest tests/unit tests/contract tests/security tests/ui: 730 passed.

After R7 (PR review): every child over A2A

New and changed: tests/unit/service/test_sub_task_brief.py (the brief text; the 4,000 character result cap), tests/unit/core/test_spawn_agents.py (description reaches the command), tests/unit/prompts/test_prompt.py (the prompt's context-isolation points), tests/unit/service/test_runtime_agents.py (self built from the harness's own card), tests/unit/test_config_agents.py (self is reserved). Each was red first. Then uv run pytest tests/unit tests/contract tests/security tests/ui: 741 passed.