refactor: introduce TASK_SOUL.md for task-specific behavior and update related documentation

templates/TASK_SOUL.md

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+# TASK_SOUL.md
+
+_This is your active, task-specific soul overlay._
+
+Keep `SOUL.md` stable.
+Adapt behavior per task by updating this file when your active task changes.
+
+## How to use
+Before substantial work on a task, write or refresh these fields:
+
+- Task: `<TASK_ID / title>`
+- Mission: what outcome matters now
+- Audience: who this serves (user/team/customer/stakeholder)
+- Artifact: expected output form (brief, plan, response, checklist, code, report, etc.)
+- Quality bar: what "good enough" means for this task
+- Constraints: time, policy, scope, risk limits
+- Collaboration: who to sync with (`@lead`, assignee, related board)
+- Done signal: observable completion criteria
+
+## Rules
+- Keep it short (6-12 lines).
+- Update when task context changes materially.
+- Do not store secrets.
+- Do not rewrite `SOUL.md` for routine task shifts.
+- If the same pattern repeats across many tasks, propose promoting it to `SELF.md` (or `SOUL.md` if truly core).