ci(extension): derive the version from commit TIME, not commit count (#3092)

Rule 149: an artifact's ordering key must be time-derived, never a commit
count. packaging.sh's cmd_patch was a count.

Why that matters here rather than in the abstract. A count is per-branch:
dev and main count different histories of the SAME code. Today only main
signs, so nothing has ordered the two against each other and the fault is
invisible. The moment dev also publishes an extension, the two versions
order by which branch accumulated more commits rather than by which is
newer — and a squash-merge makes it permanent, because main gains one
commit where dev gained five. dev then climbs away from main and a dev
install can never cross back.

That is Roundtable's 2026-08-24 incident (Scribe #2993) in a different
repo: their versionCode was the branch's commit count, and it produced a
channel you could enter and not leave. Measured on this repo today the
old formula gives main=23, dev=24 — one apart, which is exactly how the
inversion stays invisible until it strands somebody.

New formula: minutes since 2020-01-01 of the LATEST commit touching a
packaged extension file. Same anchor and unit Roundtable settled on.

Commit time, not build time, and the difference is load-bearing:

  - stable while the extension is unchanged, so the ext-<version>
    signature cache still hits and AMO is called once per extension
    CHANGE rather than once per push. Build-time minutes would re-sign
    on every push and never let two channels share a signature.
  - after a merge, main sees the same commit and derives the same
    number, so :latest reuses the signature :dev already produced for
    byte-identical code. Same code, same version, one signing.
  - monotonic: max() over a set that only gains members. Verified
    across all 24 extension-touching commits, zero non-monotonic steps.
  - reproducible from any checkout.

Derives 1.0.3499884 on dev, 1.0.3465860 on main — both far above the
last hand-set 1.0.11, so milestone 271's backfill guard is satisfied by
construction rather than by an offset.

Still shadow-only: nothing reads the derived value yet. Both shadow
steps log it, and ci.yml's runs on dev too, so both channels' numbers
are visible — that is the pair that has to stay ordered. Prior shadow
observations describe the OLD formula and prove nothing about this one,
so the window restarts; ci.yml says so at the step.

New requirement recorded in ci-requirements.md: a depth-1 clone derives
a wrong, too-low value rather than failing, so fetch-depth: 0 is
load-bearing wherever packaging.sh version is called.

Refs #3092, milestone 271

ci-requirements.md

@@ -56,11 +56,15 @@ per `docs/process.md`'s "add deps to the image when used by >1 project".
 - **`extension/scripts/packaging.sh` is the single definition of what ships
   inside the XPI.** Three consumers read from it rather than keeping their own
   copy: web-ext's `--ignore-files` (`extension/package.json`), the `:(exclude)`
-  pathspec in `ci.yml`'s `extension-version` guard, and the commit count that
-  derives the extension version. Three hand-kept copies of that one fact is
-  what allowed issue #2397.
-- `build.yml`'s `sign-extension` checks out with `fetch-depth: 0` — the derived
-  extension version is a commit count, which a shallow clone cannot produce.
+  pathspec in `ci.yml`'s `extension-version` guard, and the `git log` pathspec
+  that derives the extension version. Three hand-kept copies of that one fact
+  is what allowed issue #2397.
+- Jobs that derive the extension version check out with `fetch-depth: 0`. The
+  version is the commit TIME of the newest packaged-extension change (minutes
+  since 2020-01-01, per family rule 149 — never a commit count, which orders
+  by branch rather than by recency). A depth-1 clone sees one commit and
+  derives a wrong, too-low value rather than failing, so the full-history
+  checkout is load-bearing wherever `packaging.sh version` is called.
 - Callers MUST `set -f` before substituting the script's output. Without it the
   shell expands `test/**` against the working tree and silently narrows the
   pattern to whatever files exist at that moment — a failure that looks like