feat(commands): /update-skills syncs forks with upstream via 3-way merge

Upstream releases fixes worth pulling into the forks (arch-decide, playwright-js, playwright-py) without losing our local modifications. Each fork now has a manifest at upstreams/<name>/ plus a committed baseline snapshot that is the 3-way merge base. scripts/update-skills.py classifies each file's drift and merges to stdout. The command owns per-file confirmation, per-hunk conflict prompts, and every target write.

I centralized manifests under upstreams/ instead of per-skill dotfile dirs because arch-decide is now two flat files in commands/ and can't carry one. A "files" map in its manifest handles the upstream rename of SKILL.md to arch-decide.md.

I seeded baselines from today's upstream HEADs, so pre-existing local modifications classify as local-only from here on. git merge-file signals hard errors as exit 255, which subprocess reports as positive. The guard treats anything 128 and up as an error so a binary-file failure isn't misread as a conflict.

1 files changed, 13 insertions, 0 deletions

diff --git a/upstreams/arch-decide/manifest.json b/upstreams/arch-decide/manifest.json
new file mode 100644
index 0000000..7f781df
--- /dev/null
+++ b/upstreams/arch-decide/manifest.json
@@ -0,0 +1,13 @@
+{
+  "name": "arch-decide",
+  "url": "https://github.com/wshobson/agents",
+  "ref": "main",
+  "subpath": "plugins/documentation-generation/skills/architecture-decision-records",
+  "target": ".claude/commands",
+  "files": {
+    "SKILL.md": "arch-decide.md",
+    "/LICENSE": "arch-decide.LICENSE"
+  },
+  "license": "MIT",
+  "last_synced_commit": "cf6059d030bf4fe96623ae2e596d2f31e35fedc0"
+}