0.9.8

Brief-level task pipeline (milestone 14)

• Flattened the task pipeline to a single brief-level altitude: every entry in TASKS_TODO.md is a ## title plus a 1–3 sentence description and a trailing ---, with no Provides/Notes/Success sections, whichever path authored it.
• Replaced shared/submit-procedure.md with shared/task-format.md, holding only the brief-level template and its four authoring guidelines — referenced by both authoring runners (derive-tasks and the submit-task skill) and parsed by shared/complete-procedure.md.
• Made derive-tasks the single writer on the derivation path: it writes its ordered briefs into TASKS_TODO.md itself, retiring the per-brief dispatch loop and deleting the submit-task agent — leaving complete-task as the plugin's only remaining skill+agent pair.
• Leaned down the submit-task skill to author the same brief-level format inline and own the position-anchored insertion itself, keeping its triage, position decision, and Task-submission: commit unchanged.
• Reworked shared/complete-procedure.md to be self-sufficient at brief altitude: it derives each task's acceptance bar from the description plus requirements.md, resolves cross-task references by reading prior tasks' live deliverables, and records the derived bar in the TASKS_DONE.md entry as a **Verified:** bullet list.
• Reconciled CLAUDE.md and README.md with the flattened design and regenerated the checked-in Antigravity plugin tree under .agents/plugins/cairn/.

Full Changelog: 0.9.7...0.9.8