docs: define initial Seriatim installation feature

AGENTS.md

@@ -37,7 +37,8 @@ remotes.
   of work.
 - `docs/status.md`: current cross-repository progress and next work.
 - `docs/decisions/`: durable rationale for significant architectural choices.
-- `docs/plans/`: temporary implementation plans for substantial work.
+- `docs/roadmap/implementation.md`: the single active staged implementation
+  plan, replaced for each selected feature after its feature roadmap is agreed.
 - Application repositories: CLI, configuration, build, and integration
   contracts.
 - Individual Ansible repositories: implementation and repository-local tests.
@@ -71,12 +72,15 @@ link and a concise deployment consequence are sufficient.
 - Update `docs/roadmap/mvp.md` when a design question becomes a settled scope
   or architecture decision.
 - Discuss and select the next unit of work before planning implementation.
-- Give each selected feature a four-digit identifier and matching documents at
-  `docs/roadmap/features/<id>-<slug>.md` and
-  `docs/plans/<id>-<slug>.md`. Add the same identifier to `docs/status.md`.
+- Give each selected feature a four-digit identifier and a durable document at
+  `docs/roadmap/features/<id>-<slug>.md`. Add the same identifier to
+  `docs/status.md`.
 - Write and agree on the feature roadmap before writing its implementation
   plan. The roadmap owns scope, target end state, rationale, constraints, and
   acceptance criteria.
+- Always write the current implementation plan to
+  `docs/roadmap/implementation.md`. Replace that file when planning the next
+  selected feature; Git history preserves prior plans when needed.
 - Write implementation plans as dependency-ordered stages suitable for
   GPT-5.6-Terra to execute sequentially. Number stages with consecutive
   integers beginning with `Stage 1`; do not skip numbers or use nested stage