Add workspace-first roadmap milestones

Break the updated workspace vision into a checked-in roadmap from 2.4.0 through 3.1.0 so later implementation can be driven milestone by milestone.

Link the roadmap from the vision doc and keep each release slice scoped to one product capability, from the workspace contract pivot through shells, export/diff, services, snapshots, secrets, networking, and GA promotion.

This is a docs-only planning scaffold; runtime behavior stays unchanged in this commit.

docs/roadmap/task-workspace-ga/2.5.0-pty-shell-sessions.md

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+# `2.5.0` PTY Shell Sessions
+
+## Goal
+
+Add persistent interactive shells so an agent can inhabit a workspace instead
+of only submitting one-shot `workspace exec` calls.
+
+## Public API Changes
+
+- CLI:
+  - `pyro workspace shell open`
+  - `pyro workspace shell read`
+  - `pyro workspace shell write`
+  - `pyro workspace shell signal`
+  - `pyro workspace shell close`
+- SDK:
+  - `open_shell`
+  - `read_shell`
+  - `write_shell`
+  - `signal_shell`
+  - `close_shell`
+- MCP:
+  - `shell_open`
+  - `shell_read`
+  - `shell_write`
+  - `shell_signal`
+  - `shell_close`
+
+Core shell identity:
+
+- `workspace_id`
+- `shell_id`
+- PTY size
+- working directory
+- running/stopped state
+
+## Implementation Boundaries
+
+- Shells are persistent PTY sessions attached to one workspace.
+- Output buffering is append-only with cursor-based reads so callers can poll
+  incrementally.
+- Shell sessions survive separate CLI/SDK/MCP calls and are cleaned up by
+  `workspace delete`.
+- Keep `workspace exec` as the non-interactive path; do not merge the two
+  models.
+
+## Non-Goals
+
+- no terminal UI beyond structured shell I/O
+- no service lifecycle changes in this milestone
+- no export/diff/snapshot/reset changes yet
+
+## Acceptance Scenarios
+
+- open a shell, write commands, read output in chunks, send SIGINT, then close
+- reopen a new shell in the same workspace after closing the first one
+- delete a workspace with an open shell and confirm the shell is cleaned up
+
+## Required Repo Updates
+
+- shell-focused example in CLI, SDK, and MCP docs
+- help text that explains shell vs exec clearly
+- real Firecracker smoke for open -> write -> read -> signal -> close