feat(firmware): add persistent-clone git sync spike (git_sync)

Milestone #2 increment A: the product's real publish flow, distinct from
git_push's fresh-init-per-boot throwaway branch. open-or-clone a persistent
/spiflash/repo, append to a tracked notes.md, commit on top of the branch,
and fast-forward push over mbedTLS HTTPS+PAT. Reconcile handles up-to-date
and fast-forward; a true divergence (merge commit on FATFS) is deferred to
increment B. Runs on the dedicated 96 KB git thread; gated behind `git`.

Compiles clean; on-device clone/open+fast-forward not yet hardware-verified
(clone/checkout is deeper than the proven init path — the flash may need a
GIT_STACK bump).

firmware/justfile

@@ -12,6 +12,7 @@ elf_wifi := "target/xtensa-esp32s3-espidf/release/wifi_tls"
 elf_sd := "target/xtensa-esp32s3-espidf/release/sd_fat"
 elf_git := "target/xtensa-esp32s3-espidf/release/git_smoke"
 elf_git_push := "target/xtensa-esp32s3-espidf/release/git_push"
+elf_git_sync := "target/xtensa-esp32s3-espidf/release/git_sync"
 
 # Custom partition table (adds the `storage` FAT partition for the git working
 # copy). Only the git-push flash applies it — the editor flash keeps the default
@@ -92,6 +93,19 @@ flash-git-push: build-git-push
 monitor-git-push:
     espflash monitor --elf {{elf_git_push}}
 
+# Milestone #2A — build the persistent-clone publish cycle (clone + fast-forward push)
+build-git-sync:
+    {{esp_env}} {{git_env}} cargo build --release --bin git_sync --features git
+
+# Milestone #2A — flash + monitor. Applies the same custom partition table as
+# git-push (the `storage` FAT partition holds the persistent clone).
+flash-git-sync: build-git-sync
+    espflash flash --monitor --partition-table {{partition_table}} --flash-size 16mb {{elf_git_sync}}
+
+# serial monitor for the git sync, with decoded backtraces
+monitor-git-sync:
+    espflash monitor --elf {{elf_git_sync}}
+
 # detect board, print chip/MAC/flash size
 info:
     espflash board-info