fix(envd): default bash CWD to workspace root + never surface a bare EOF

[ysyneu]

Why

Two distinct failures from prod AI-SRE session sess_f2NLHa2qUpTnUnJRJmSMp3, where the agent repeatedly failed to author a knowledge file in a cloud sandbox.

1. Bash CWD diverged from the file-tool root (the real root cause)

handleProcessStart left cmd.Dir unset when the client omitted cwd, so a bare bash inherited envd's own process cwd — the image WORKDIR (e.g. /home/runner) — while the file tools (read/write/glob/sync_files) root at WorkspaceRoot (e.g. /home/runner/.flashduty).

Consequences the agent actually hit:

• files written via bash (echo > foo.md, cd-relative paths) landed in /home/runner and were invisible to read/glob;
• absolute paths the agent computed from bash pwd were rejected by the file tools as path escapes environment root.

The BYOC exec path already does the right thing (resolveWorkdir → e.root). This change makes the cloud path match: an empty cwd now defaults to WorkspaceRoot.

2. Opaque EOF hid every write/tool failure

writeUnary emitted an empty body on success-with-no-payload (e.g. Write). The safari client decodes every unary response with json.Decode, and an empty body decodes to io.EOF — which surfaced to the agent as the meaningless remote write operation failed: EOF, masking the real cause.

Now:

• every unary response is always a complete JSON envelope (nil payload → {});
• write errors are returned verbatim with the path;
• a recoverMiddleware turns handler panics into proper error envelopes instead of a dropped connection (which the client would also see as a bare EOF), re-panicking http.ErrAbortHandler so net/http's own abort semantics are preserved;
• startup logs the resolved workspace root, so a future root mismatch is greppable.

Tests

• TestProcessStart_DefaultsCwdToWorkspaceRoot — bare bash with no cwd resolves a relative file against WorkspaceRoot.
• write_eof_test.go — Write and error paths always return a decodable envelope, never EOF.
• connect_test.go — panic in a handler becomes an error envelope; http.ErrAbortHandler re-panics.
• go build ./... clean, go test ./envd/... ./environment/... ./cmd/... green, lint clean (only the pre-existing broker_other.go unused warning on main).

Companion PRs (same incident)

• fc-safari: cloud RemoteEnvironment reads envd's real workspace root + clearer path errors (fix/cloud-workspace-root → feat/ai-sre).
• flashduty-mcp-server: query_incidents optional time window (fix/query-incidents-time-window → main).