docs: split roadmap out of readme into Documentation/roadmap.md

The readme served two roles: design-decision source of truth and
work-status tracker. The status section grew into a plan the
orchestrator must navigate, so it moves to its own file with task
IDs, dependency edges, statuses, and per-item acceptance criteria.

readme.md remains authoritative for decisions (stack, formats,
workflow); workflow step 7 and AGENTS.md pointers now name
roadmap.md for status. Cross-references verified.

Also records: stale feature/epsilon-parser branch pointer (676e55e,
2 behind main) deleted locally; recreate from main when B1 starts.

AGENTS.md

@@ -1,12 +1,14 @@
 # AGENTS.md — working rules for coding agents on RustyRPN
 
-readme.md is the single source of truth for design decisions and roadmap.
-Read at minimum these sections before starting work:
+readme.md is the single source of truth for design decisions. The
+roadmap lives in Documentation/roadmap.md.
+Read at minimum these before starting work:
 
-- §Workflow — the TDD loop you are expected to follow
-- §General guidelines for AI — hard prohibitions
-- §Data formats — input formats, edge cases, and where test fixtures live
-- §Roadmap and status of functionality — what is green, red, or unstarted
+- readme.md §Workflow — the TDD loop you are expected to follow
+- readme.md §General guidelines for AI — hard prohibitions
+- readme.md §Data formats — input formats, edge cases, and where test
+  fixtures live
+- Documentation/roadmap.md — what is green, red, or unstarted
 
 ## Environment
 
@@ -18,17 +20,18 @@ Read at minimum these sections before starting work:
   `src/cli` (rpn-cli, binary `rpnc`). `src/server` and `src/web` are
   planned but not scaffolded yet.
 
-## Current state (verify against readme.md roadmap; update both when it changes)
+## Current state (verify against Documentation/roadmap.md; update both when it changes)
 
 - core/config: implemented, tests green.
-- core/db: tests committed and RED (they reference functions that do not
-  exist yet — the crate does not compile its test target). The first task
-  is the minimal implementation to turn them green. Do not weaken or
+- core/db: tests committed and RED (they reference functions that do
+  not exist yet — the crate does not compile its test target). The first
+  task is the minimal implementation to turn them green. Do not weaken or
   rewrite these tests to make them pass; they encode agreed behaviour.
 - cli: `fn main() {}`. Command surface specified in Documentation/cli.md.
-- feature/epsilon-parser: branch exists, zero commits. Needs workflow
-  step 2 (spec + test checklist agreed with the maintainer) before any
-  failing test is written.
+- feature/epsilon-parser: does not exist yet. Create it from `main`
+  when parser work starts (a stale early pointer was deleted
+  2026-10-08). Workflow step 2 (spec + test checklist agreed with the
+  maintainer) must complete before any failing test is written.
 
 ## Test data
 
@@ -58,6 +61,7 @@ Read at minimum these sections before starting work:
    `cargo clippy` clean for files you touched.
 2. Every checklist item from the task's spec verified — by a test or by
    a command you actually ran, not by inspection alone.
-3. readme.md roadmap updated in the same branch.
+3. Documentation/roadmap.md updated in the same branch (and readme.md
+   if a design decision changed).
 4. A short summary for the maintainer: what changed, what is still red,
    suggested next step.