fix(engine): BUG-6 — approved writes must land, and say where (false-receipt kill)

Two compounding defects made every pause->approve write_file report success
while writing NOTHING:

1. The naive json_get scanner matches "content" anywhere in the approve
   body — including INSIDE tool_input, which for write_file always carries
   a content field. The handler therefore treated every approved builtin
   write as already-client-executed, skipped dispatch entirely, and handed
   the model the file's own content as the 'tool result'. The model then
   narrated 'Done, created' — a false receipt with no file. Builtin tools
   now ALWAYS dispatch server-side; client content is only honored for
   non-builtin (MCP/client-executed) tools. Stricter only.

2. write_file returned {"ok":true} unconditionally — fs_write's outcome
   was never checked, so any failed write also reported success. The write
   now verifies the file landed (fs_exists) and returns the RESOLVED path
   in the ok payload; failures return a real error naming the destination.

E2E on the test brain (boot 38): approve-path write lands byte-exact and
the result carries the resolved path; auto-run writes unchanged; denied
writes execute nothing. BUG-5 (approve wire lacked tool_name) had been
masking this one — two stacked bugs on the same path.

NOTE for review: the deeper cure is a nesting-aware json reader; this fix
removes the dangerous consequence at the two spots that lie about disk.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

sessions.el

@@ -713,7 +713,13 @@ fn handle_session_approve(session_id: String, body: String) -> String {
         // For builtin tools with no client-provided content: fall back to
         // dispatch_tool so those tools still execute correctly.
         let client_content: String = json_get(body, "content")
-        let use_client_content: Bool = !str_eq(client_content, "")
+        // BUG-6 fix (2026-07-17): the naive json_get scanner matches "content" ANYWHERE
+        // in the body — including INSIDE tool_input — so every approved write_file (whose
+        // input always carries a content field) was mistaken for client-executed, never
+        // dispatched, and narrated as done: a false receipt with no file on disk. Builtin
+        // tools now ALWAYS dispatch server-side; client content is only honored for
+        // non-builtin (MCP/client-executed) tools. Stricter only.
+        let use_client_content: Bool = !str_eq(client_content, "") && !is_builtin_tool(approve_tool_name)
         let use_dispatch: Bool = is_builtin_tool(approve_tool_name) && !use_client_content
         let raw_input: String = json_get_raw(body, "tool_input")
         let eff_input: String = if str_eq(raw_input, "") { "{}" } else { raw_input }