fix(missions): a coding phase that delivers nothing is a failure

The last open item in the silent-success class: a coding phase that
changed no files reported `completed` — the same status a phase gets for
delivering tested, reviewed, pushed work. Mission `019fcf62` completed
that way with its agents silently unpinned from the repo, and nothing in
the platform disagreed; it was found by a script diffing the forge.

The verdict is applied at capture rather than at completion, because
capture selects on `status = 'completed'` — the platform does not know
whether a phase produced anything until after it has already finished.

Three conditions must hold before failing a phase, because a false
positive here fails honest work: the phase is a coding phase (research
phases legitimately write nothing to the tree), the diff was actually
computed (an uncomputable diff also reports zero files — blaming the
agent for a platform fault is the same defect wearing different
clothes), and `allow_empty` is not set. Only an explicit `true` opts
out, so a typo leaves the check armed. Registered in phase_config with
its reader named, per the seam-2 rule.

Also closes an ordering hazard this exposed: capture is batched and runs
after a phase completes, so a backlogged mission could close as
'completed' and only then have capture discover an empty phase — leaving
a 'completed' mission holding a 'failed' phase, unfixable because the
mission-close CASE only touches 'running' rows. A repo-bearing mission
now waits for its work to be captured before closing.

Adds a `noop` scenario to the harness: a phase told to change nothing,
which PASSES only when the phase comes back `failed`. Same discipline as
the uid self-test — a check that has never been seen to fire has not
been shown to work.

Co-Authored-By: Claude Opus 5 <[email protected]>

crates/cm-api/src/mission_delivery.rs

@@ -109,6 +109,10 @@ pub struct Capture {
     /// The phase changed nothing. Still recorded — "this coding phase wrote no
     /// code" is currently invisible to an operator, and it is worth saying.
     pub empty: bool,
+    /// Why the diff could not be computed, if it could not be. `empty` is only
+    /// meaningful when this is `None`: otherwise the tree was never read, and
+    /// callers deciding anything on the strength of "no changes" must not.
+    pub diff_error: Option<String>,
     pub truncated: bool,
     pub patch_path: PathBuf,
     /// Set once the work has been committed to a mission branch.
@@ -463,6 +467,7 @@ pub async fn capture_phase_diff_at(
         insertions,
         deletions,
         empty,
+        diff_error,
         truncated,
         patch_path,
     }))