feat: workspace browser with --workspace-root opt-in scoping (#526)

* feat(web): add workspace browser for multi-directory navigation

Add /browse route with a folder browser that lets users navigate
filesystem directories on connected machines and launch sessions
from any folder. Supports saved workspace paths and direct path
input. The "Start Session" action pre-fills the NewSession form.

- CLI: register machine-level `list-directory` RPC handler
- Hub: add POST /machines/:id/list-directory route
- Web: add WorkspaceBrowser component with git repo detection
- Web: add /browse route with navigation from sessions sidebar
- Web: support initialDirectory/initialMachineId in NewSession

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: add --workspace-root opt-in scoping for /browse and session spawn

Adds a single new flag, \`--workspace-root <path>\` (with \`~\` / \`~/foo\`
expansion), on \`hapi runner start\` and \`hapi runner start-sync\`.

When set:
- The runner reports the path in machine metadata.
- The list-directory and spawn-session RPC handlers reject paths outside
  the root, so the web UI can't escape the configured tree even if
  someone crafts a request manually.
- The /browse page in the web UI auto-opens that root, restricts the
  breadcrumb / go-up to its subtree, and shows directory entries with
  git-repo annotations.
- The /sessions/new form keeps its existing free-text directory input
  plus autocomplete + recent-paths chips, and gains a small "Browse"
  button (next to the input) that opens /browse for picking a folder.
- Reconnect-time metadata sync ensures stale records get the field
  filled in (or cleared when the flag is dropped on a later restart),
  so the hub state matches the CLI's intent.

When unset:
- Runner behaves like the legacy hapi (no scoping, no browse feature).
- /browse renders an informative state pointing at the flag instead of
  blocking the user.
- The /sessions/new form looks identical to the pre-change behavior;
  the "Browse" button is hidden.

Includes a startup banner so \`runner start-sync\` no longer looks like
it hung, and surfaces the workspace-root sync result on stdout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(hub): preserve workspaceRoot when rehydrating machines from store

MachineCache.refreshMachine() rebuilt the metadata object from an
explicit field allowlist, so any field not in the list (including the
new workspaceRoot) was silently dropped on every read — even though it
was correctly written to the store.

Add workspaceRoot to the zod schema, the Machine interface, and the
hand-rolled projection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* feat(web): friendlier empty state on /sessions

When there are zero sessions the page used to be a vast blank
rectangle with just the "0 sessions in 0 projects" caption. Render a
centered empty state instead: a calendar/agenda icon, a short heading
and hint, and two buttons — "Start a session" (→ /sessions/new) and
"Browse workspace" (→ /browse).

SessionList gains an optional onBrowse prop. Router wires it on the
sessions page so the secondary button resolves; other callers can leave
it unset to hide that button.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs: document --workspace-root flag in cli/README and root README

Add a short paragraph under "Runner management" in cli/README.md
explaining what \`--workspace-root\` enables (scoped /browse tree,
list/spawn enforcement, tilde expansion) and that omitting it keeps
the legacy behavior. Mention the workspace browser in the top-level
README's Features list.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix: address PR #526 review feedback

Three findings from the review bot:

1. [Major] Workspace-scope check was lexical only. With workspaceRoot
   = /safe, a symlink such as /safe/out -> /etc would pass the relative-
   path test and let list-directory / spawn-happy-session reach paths
   outside the configured root. realpath the workspaceRoot at construction
   time, and resolve every incoming path through realpath (walking up to
   the nearest existing parent for spawn targets that haven't been
   created yet) before the containment check.

2. [Minor] \`hapi runner start --workspace-root\` with no value used to
   drop the flag silently and start the runner unscoped. Now treats a
   missing or flag-shaped next argument as an error.

3. [Minor] /sessions/new's "Browse" button always opened /browse using
   localStorage's last-used machine, ignoring the user's current
   selection. NewSession already passes machineId in its callback;
   forward it through the /browse search params and seed
   WorkspaceBrowser with it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(cli): gate list-directory RPC behind --workspace-root opt-in

Without a configured workspaceRoot, isWithinWorkspaceRoot() returns
true unconditionally, leaving the new list-directory RPC able to
enumerate any path on the runner. The Web UI already hides Browse
for these machines, but the backend should enforce the opt-in too.

Refuse the RPC up front when no workspace root is configured.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -12,6 +12,7 @@ Run official Claude Code / Codex / Gemini / OpenCode sessions locally and contro
 - **Your AI, Your Choice** - Claude Code, Codex, Cursor Agent, Gemini, OpenCode—different models, one unified workflow.
 - **Terminal Anywhere** - Run commands from your phone or browser, directly connected to the working machine.
 - **Voice Control** - Talk to your AI agent hands-free using the built-in voice assistant.
+- **Workspace Browser** - Opt-in via `hapi runner start --workspace-root <path>`: browse a scoped file tree from the web and start sessions in any subdirectory.
 
 ## Demo