Filesystem watch (Phase 2): streaming WatchDir - #3

Summary

Adds live directory watching to E2bEx.Filesystem via the envd streaming WatchDir RPC — message-first, like Commands.start/PTY.

• E2bEx.Filesystem.watch_dir/4 → {:ok, %WatchHandle{}}, pushing {ref, {:fs_event, %E2bEx.FilesystemEvent{}}} to a subscriber as changes happen; ends with a terminal {ref, {:error, %E2bEx.Error{}}} if the stream fails or closes.
• E2bEx.FilesystemEvent — {name, type (:create|:write|:remove|:rename|:chmod), entry} (the %EntryInfo{} is included when include_entry: true).
• E2bEx.Filesystem.WatchHandle — stop/1 ends the watch (no terminal message), cancelling the stream.
• E2bEx.Filesystem.WatchServer — internal streaming GenServer, modeled on Commands.HandleServer. The key difference: WatchDir frames are a bare WatchDirResponse oneof (start/filesystem/keepalive), not wrapped in an "event" field; and watch has no result, so a closed stream is terminal {:error, …}.

opts: :recursive, :include_entry, :subscriber, plus the usual :user/:timeout_ms/:domain/:port/:base_url.

Notes

• Stacked PR: based on feat/filesystem (PR Filesystem Phase 1: file operations + content #2, Phase 1). Once Filesystem Phase 1: file operations + content #2 merges to main, this rebases onto main. Phase 1 code is untouched here — only watch_dir/4/spawn_watch/3 were added to filesystem.ex.
• The streaming-skeleton duplication with HandleServer is a deliberate, documented choice (deferred unification) to avoid touching the shipped Commands/PTY path.
• Out of scope: the polling watcher RPCs (CreateWatcher/GetWatcherEvents/RemoveWatcher).

Spec: docs/superpowers/specs/2026-06-13-e2b-filesystem-watch-design.md · Plan: docs/superpowers/plans/2026-06-13-e2b-filesystem-watch.md

Test Plan

• mix test — 163 tests, 0 failures
• mix compile --warnings-as-errors clean
• Coverage: FilesystemEvent.from_api/1 (all event types + nested entry); WatchHandle.stop/1 (live + dead); watch_dir/4 streaming (chunked incremental decode, entry present/absent, keepalive-ignored, defaults, pre-start non-2xx error, mid-stream trailer error, post-start truncated-frame malformed, stop/1 no-terminal)
• Manual: watch a real sandbox dir and touch/edit/remove files

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