dashboard: diff against applied/proposal/<id>, prefer fetched_sha

approval_diff now runs git diff refs/heads/main..refs/tags/
proposal/<id> against the applied repo instead of cobbling a
single-file diff from proposed. consequences: multi-file
proposals show every change, manager amendments in proposed
cannot lie about what'll be deployed, no-op proposals render
an explicit '(proposal matches currently-deployed tree)'.
displayed sha prefers fetched_sha (hive-c0re-vouched) and
falls back to commit_ref only for the brief pre-fetch window.
unified_diff helper + similar dep dropped — git diff is the
source of truth now. dead-code allows on the lifecycle git
helpers + approvals.set_fetched_sha come off since all are
wired up. readme picks up the tag flow + /applied RO mount.

Cargo.lock

@@ -474,7 +474,6 @@ dependencies = [
  "rusqlite",
  "serde",
  "serde_json",
- "similar",
  "tokio",
  "tokio-stream",
  "tracing",
@@ -986,12 +985,6 @@ dependencies = [
  "libc",
 ]
 
-[[package]]
-name = "similar"
-version = "2.7.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bbbb5d9659141646ae647b42fe094daf6c6192d1620870b449d9557f748b2daa"
-
 [[package]]
 name = "slab"
 version = "0.4.12"