docs: Add ACP roadmap

ROADMAP.md

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+# Nori CLI ACP Roadmap
+
+This roadmap is about Nori's ACP client work, not ACP in the abstract. The goal is to keep the client simple, fast, and predictable while we fill in the highest-value protocol features.
+
+## Current Focus
+
+### Core Session Lifecycle
+
+- DONE: `load`, `list`, `resume`, and `undo`
+- TODO: `fork` and tree-oriented session flows
+- TODO: hook resume up to a top-level subcommand
+- TODO: improve resume performance
+
+This is the main workstream right now. Session lifecycle is the foundation for everything else, so we want it to feel complete before we spread effort across more protocol surface.
+
+## Next Features
+
+These are the next highest-leverage additions after the core lifecycle work. They make Nori more useful without adding much conceptual weight.
+
+### Session Configuration
+
+- custom config options such as thinking or effort level
+- mode-style options such as plan versus build
+
+### Agent Discovery
+
+- support the official ACP Registry for discovering custom agents
+
+## Future Features
+
+### Agent-Driven Auth
+
+- auth methods
+- logout
+
+We want these, but they are behind lifecycle, configuration, and registry support.
+
+## Already Working Well
+
+- session loading
+- local session continuity and resume flows
+- undo support
+- custom agent registration through local config
+
+These areas are good enough to build on, even where we may still want cleaner or more native ACP implementations underneath.
+
+## ACP Feature Reference
+
+This section is the dependency-facing catalog. It exists to keep the roadmap grounded in the current ACP spec and draft surface, but it is not the main structure of the project roadmap.
+
+| Feature                         | ACP status                | Nori status           |
+| ------------------------------- | ------------------------- | --------------------- |
+| `session/load`                  | stable baseline           | done                  |
+| `session/list`                  | stable                    | done                  |
+| Session Config Options          | stable                    | near term             |
+| `session_info_update`           | stable                    | later                 |
+| ACP Registry                    | stable                    | near term             |
+| `session/resume`                | unstable landed           | done                  |
+| `session/fork`                  | unstable landed           | todo                  |
+| `session/close`                 | unstable landed           | handled               |
+| `additionalDirectories`         | unstable landed           | partial               |
+| auth methods                    | unstable landed           | planned               |
+| `logout`                        | unstable landed           | planned               |
+| `messageId` and `userMessageId` | unstable landed           | partial               |
+| usage updates                   | unstable landed           | partial               |
+| boolean config options          | unstable landed           | near term             |
+| elicitation                     | unstable landed           | later                 |
+| NES                             | unstable landed           | later                 |
+| `session/delete`                | draft only                | not planned near term |
+| custom provider endpoints       | draft only                | not planned near term |
+| diff-delete metadata            | draft only                | not planned near term |
+| proxy-chains                    | draft only                | not planned near term |
+| subagents                       | not first-class ACP today | not planned near term |
+
+## Notes
+
+- This reflects our current understanding as of April 2026.
+- "Done" in this roadmap means done enough for Nori's current product goals, not necessarily that every ACP-native implementation detail is perfect.
+- We should keep preferring fewer special cases and more native ACP behavior as we iterate.