fix: surface store sync failures + add secrets sync (EGB-1230, EGB-1231)

EGB-1230: `cmd_pull` synced the store with `git pull >/dev/null 2>&1` under
`set -euo pipefail`. A store that couldn't fast-forward killed the script at
that line with git's exit 128 and nothing on stdout or stderr — a banner, no
restored files, no reason, and invisible in a pipeline. The sync now routes
through `_store_sync_pull`, which guards the pull, captures git's output as
the diagnosis, and dies naming the store path and `secrets sync`. It is now
`--ff-only` to match the push path, so a plain pull can no longer manufacture
a merge commit in the store.

EGB-1231: once a store diverged there was no way out — push demanded a
fast-forward and pointed at pull, which couldn't fast-forward either, so the
advice looped and recovery meant hand-running git next to encrypted blobs.
Adds `secrets sync`: fetch, stash, rebase onto the remote, restore the stash,
then a confirmation-gated push of local commits (`--yes` to skip the prompt,
`--dry-run` to report only). Non-destructive by construction — no merge, no
force-push, no `reset --hard`, no `stash drop`; a rebase conflict names the
conflicting files and leaves the store exactly as found. `secrets which` gains
a `remote:` line reporting ahead/behind/dirty, and push's dead-end message now
points at `sync`.

test/sync.bats: 25 new tests. Full suite 353/353 green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BrUoYuUMoTj91rzV4vxGPB

VERSION

@@ -1 +1 @@
-0.7.5.0
+0.7.6.0