docs(agents): add Task flow section
Replaces the one-line 'Task lifecycle' pointer with a full Start / During /
End flow that consolidates the rules scattered across the rest of AGENTS.md
(Authoring boundaries, Working style, Releases, Decision Graph) into one
sequence the agent can follow turn-by-turn. The procedural detail still
lives at .opencode/skills/task-lifecycle/SKILL.md; this section is the
in-context summary.
Start: pull main, branch, log goal, design in plan mode.
During: action nodes, process skills, authoring boundaries, commits,
deciduous linking.
End: verify, task note, version bump, hand back.
Co-authored-by: opencode <noreply@opencode.ai>