Refactor sprint planning docs and add templates

Updated AGENTS.md with implementation planning conventions and stream index. Refactored SPRINT_110_ingestion_evidence.md, SPRINT_125_mirror.md, and SPRINT_300_documentation_process.md to use a topic-oriented template, clarify dependencies, task boards, and checkpoint structure. Archived previous sprint details and added new templates and status snapshot files to docs/implplan.

docs/implplan/templates/sprint-template.md

@@ -0,0 +1,28 @@
+# Sprint <ID> · <Stream/Topic>
+
+## Topic & Scope
+- Replace with a 2–5 bullet summary of what this sprint delivers and why it matters.
+
+## Dependencies & Concurrency
+- Upstream sprints or artefacts that must land first.
+- Confirm peers in the same decade (e.g. 120–129) remain independent.
+
+## Documentation Prerequisites
+- List onboarding docs, architecture dossiers, runbooks, ADRs, or experiment notes that contributors must read before flipping any task to `DOING`.
+
+## Task Board
+| Task ID | Status | Owner(s) | Dependencies | Notes |
+| --- | --- | --- | --- | --- |
+| EXAMPLE-00-001 | TODO | Guild · Team | Upstream contract or sprint | Replace with the real backlog. |
+
+## Execution Log
+| Date (UTC) | Update | Owner |
+| --- | --- | --- |
+| 2025-11-13 | Example entry describing what moved. | Guild |
+
+## Decisions & Risks
+- Pending approvals, blocked schema reviews, or risks with a mitigation plan.
+
+## Next Checkpoints
+- Dated meetings, demos, or cross-team alignment calls with the accountable owners.
+