Use each xmd repl session directory as its workspace #975

Story

As a person working with the Sidekick in xmd repl, I want the existing session directory to be its workspace, so components, copied inputs and generated files stay with the conversation and relative paths have a predictable starting point.

Common path

A session already has a directory containing its REPL and Sidekick journals. Startup also provides components/ inside that directory and makes those components available automatically.

The Sidekick writes components/Greeting.md; a subsequent entry invokes <Greeting /> without a manual include option or restart. XMD executed by the Sidekick uses the session workspace as its default working directory: writing notes.md or copying an input to inputs/request.md places those files beneath that session directory. The host establishes this default through XMD's existing contextual working-directory mechanism, as <Dir> does.

Reopening the session finds the same workspace, components, files and journals. A different session has its own workspace.

Current gap

The session directory stores conversation history but is not the default filesystem location described above. #896 owns creating, testing and reusing Sidekick components and leaves the directory's location and lifetime for a product decision. It does not establish the broader session workspace or the default working directory for Sidekick-executed XMD.

Accepted direction

• Reuse the existing session directory as the workspace rather than creating a separate temporary artifact root. Establish it and its components/ directory before the first Sidekick evaluation or entry that uses them.
• Include the workspace components directory automatically in ordinary component discovery, validation and execution. Definitions written before a subsequent entry are available to it; writing a component alone does not execute it.
• Establish the workspace as the contextual working directory around Sidekick XMD evaluation, using the same scoped mechanism as <Dir>. Relative file reads, writes and copy destinations use that directory. An explicit nested <Dir> selects another authorized directory for its content; the enclosing directory is restored when it finishes. Success, failure and cancellation do not leak the session cwd into sibling or caller scopes. No new REPL.Environment field or workspace-discovery component is required.
• Keep session-owned components, imported inputs, generated documents, handoffs, results and evidence in the workspace by default. This directory convention does not automatically ingest every file into a journal or make mutable files historical snapshots.
• Preserve the workspace across exit and cold reopen. Resume the same session with its existing identities and recorded execution; completed operations are not rerun merely to rebuild history.
• Preserve the originally selected project repository, worktrees and CLI path arguments when changing the default execution directory. Resolve project selection and invocation-relative options deliberately rather than accidentally treating the session directory as the project. Do not move or reset existing source checkouts or replace agent sessions as a side effect.
• Preserve existing permission and component-name protections. A new working directory or include path grants no additional effects or filesystem access.

Decisions before implementation

• Consider moving the REPL and Sidekick journals into a dot directory beneath the workspace to separate host-owned history from authored files. Its name and exact layout remain open. Define compatibility for existing sessions before moving any journal; preserve their complete histories.
• Settle whether the workspace working-directory default also applies to every human-authored REPL entry. Sidekick evaluation and submitted-entry behavior must be explicit, without an accidental difference between validation and execution.
• Settle include precedence with project components and --include, and behavior when existing workspace files are missing or malformed. Coordinate these decisions with Create, test and reuse Sidekick components in xmd repl #896 rather than creating a second component-authoring interface.

First bounded implementation slice

Set the existing session directory as the contextual cwd for Sidekick-executed XMD. Verify a relative file read/write lands there, a nested <Dir> uses its selected directory, and scope teardown restores the prior cwd on success, failure and cancellation. This slice reuses existing directory context; it introduces no environment-discovery interface.

This is independently testable before component-authoring integration or journal relocation. It does not settle the cwd default for human-authored ordinary entries. The complete story remains open until its other accepted requirements are delivered.

Acceptance and evidence

Use isolated session storage and temporary project repositories. Start a fresh production REPL without creating directories or supplying --include; observe its workspace and components directory. Create a component, invoke it in a later entry, write a relative document and copy a project input into the workspace. Confirm the exact destinations and returned contents. Confirm that the selected project repository and invocation-relative include paths still refer to the intended project.

Exit and reopen the session; the same files and component are present. A second session does not share them by default. Inspect completed history and show zero repeated file mutations or agent calls. Preserve existing journals and user files through the chosen old-session compatibility path. A failed component write or malformed component reports its actual failure without silently executing another definition.

Relationships and scope

Coordinate with #896 for automatic component availability and its authoring/testing journeys, #968 for external include paths, and #901 for session-owned TODO files and policy components. This is a REPL session workspace story; #218's provider-backed retained workflow Workspace is a separate contract. Update the REPL specification and agent-facing environment guidance when this behavior is delivered.

This story supplies the default session workspace and working-directory behavior. Ticket orchestration, artifact versioning and changes to workflow approval policy retain their separate scope.