fix(web-shell): isolate history and session drafts

docs/design/web-shell-workspace-history-session-drafts.md

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+# Web Shell workspace history and session drafts
+
+## Problem
+
+The prompt history currently uses one browser storage key for every workspace.
+Its navigation cursor also survives session changes when the editor component
+is reused. Unsent editor text is not persisted, so it is lost when a session is
+left or the editor remounts.
+
+Queued prompts already enter history through the composer's accepted-submit
+commit callback on current `main`. Adding another write in the queue layer
+would duplicate that behavior.
+
+## Design
+
+- Derive the prompt-history storage key from the effective workspace cwd.
+  Reload the in-memory history and reset navigation whenever that key changes.
+  If a workspace has no scoped history yet, read the legacy unscoped history
+  and migrate it into the workspace key on the next accepted prompt.
+- Pass the active session id and effective workspace cwd through `ChatEditor`.
+- Reset prompt and shell history navigation whenever the session or workspace
+  changes.
+- Persist plain editor text under a session-specific browser storage key. On
+  the New Task page, where no session exists yet, use a workspace-specific
+  pending-task key instead. Keep the draft in memory until the workspace is
+  known so an unscoped key cannot leak text between workspaces. Restore that text
+  when its session or New Task workspace returns. Debounce writes by two
+  seconds, updating only a deadline on the input path and reading the editor
+  when the timer expires. Flush pending text before switching identity,
+  unmounting, or leaving the page. Remove the source draft after an accepted
+  submission only when the composer still contains the submitted snapshot.
+- Keep the existing accepted-submit callback as the only history commit point,
+  including the first prompt that lazily creates a session, with regression
+  coverage for delayed acceptance. A callback that completes after the user
+  switches sessions writes only to its source history and never clears the
+  newly active composer.
+
+Composer tags, images, and other transient editor UI state are not persisted by
+this change.