docs(skill): teach this season's flow anti-patterns in the automation authoring skill (#1921)

Upstream root-cause fix for AI authoring mistakes: the templates are AI-written,
so the highest-leverage prevention is teaching the AI the correct patterns at the
source, not only catching them at build.

Adds a "Valid-but-silently-wrong (passes build, fails at runtime)" subsection to
skills/objectstack-automation Common Pitfalls, capturing the anti-patterns fixed/
linted this season, each with ❌/✅:
- single-brace value interpolation; no `{{double}}` / bare `$ref` (#1315)
- `create_record` outputVariable is the RECORD → `{var.id}` (#1873)
- time-relative rules = schedule + range query, NOT record-change date-equality (#1874)
- `script` nodes must name a built-in actionType or a registered `function`;
  inline `config.script` is not executed (#1870)
- conditions are bare CEL using only the stdlib; unknown fns fail the build;
  don't wrap field refs in `{…}` (#1877/#1491)

Docs-only; `check:skill-docs` passes (frontmatter-derived docs unaffected); empty changeset.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: os-zhuang

.changeset/automation-skill-antipatterns.md

@@ -0,0 +1,13 @@
+---
+---
+
+docs(skill): add "valid-but-silently-wrong" anti-patterns to the automation authoring skill
+
+Documentation-only — no package change. Adds this season's flow authoring
+anti-patterns (the ones that pass build but fail at runtime, and that the new
+build lints now catch) to `skills/objectstack-automation` Common Pitfalls, so the
+AI author writes them right at the source: single-brace value interpolation
+(#1315), `create_record` outputVariable holds the record / `{var.id}` (#1873),
+time-relative rules as schedule+range not record-change date-equality (#1874),
+`script` nodes must name a callable (#1870), conditions are bare CEL / stdlib only
+(#1877).

skills/objectstack-automation/SKILL.md

@@ -511,6 +511,50 @@ read at runtime, not Zod-validated):
 5. **Scheduled flows without idempotency.** If the flow runs twice
    accidentally, the result should be the same.
 
+### Valid-but-silently-wrong (passes build, fails at runtime)
+
+These are *legal* metadata that authors — AI especially — get wrong. Most are now
+caught by `objectstack build` (a hard error, or an advisory warning), but write
+them right the first time:
+
+6. **Flow node VALUE interpolation uses SINGLE braces.** Value fields on a node's
+   `config` (`fields`, `inputs`, notify `message`/`title`, …) interpolate
+   `{token}`:
+   - `{var}` / `{record.title}` — variable / record field
+   - `{record.tags.0}` — **array index** (e.g. a `multiple: true` lookup, stored as an array)
+   - `{$User.Id}` / `{NOW()}` / `{TODAY() + 30}` — current user / date macros
+   - anything without `{…}` is a **literal**
+
+   ❌ `body: '{{ai_reply}}'` — double-brace is the *formula / template-field* dialect, **not** flow values
+   ❌ `ticket: '$source.id'` — a bare `$ref` is a literal string, not interpolated
+   ✅ `body: '{ai_reply}'`, `ticket: '{source.id}'`
+
+7. **`create_record`'s `outputVariable` holds the created RECORD, not its id.**
+   Reference a field explicitly.
+   ❌ `update_record … fields: { ref: '{newRec}' }` → yields the whole record object
+   ✅ `fields: { ref: '{newRec.id}' }`
+
+8. **Time-relative rules ("alert N days before a date") are SCHEDULE flows, not
+   record-change date-equality.** `record.end_date == daysFromNow(60)` on a
+   `record-*` trigger only fires if the record happens to be written on that exact
+   day — unattended rules never run.
+   ✅ A daily `schedule` flow whose `get_record` filters `end_date` BETWEEN
+   `{TODAY()}` and `{TODAY() + N}`, then loops over the results.
+
+9. **`script` nodes must name a callable.** Set `config.actionType` to a built-in
+   side-effect (`email` / `slack`) **or** `config.function` to a function
+   registered via `defineStack({ functions: { my_fn: (ctx) => … } })`. An empty
+   `script` node — or one pointing at an unregistered function — fails loudly.
+   Inline `config.script` JS is **not executed** by the built-in runtime (no
+   server-side sandbox) — move logic into a registered `function`.
+
+10. **Conditions are bare CEL — only the stdlib is callable.** `now()`,
+    `today()`, `daysFromNow(n)`, `daysAgo(n)`, `isBlank(v)`, `coalesce(a, b)`,
+    `trim(s)`, plus CEL built-ins (`has`, `size`, `contains`, `startsWith`, …).
+    An UNKNOWN function (`PRIOR()`, a typo'd name) **fails the build**. And never
+    wrap a field reference in `{…}` inside a condition — that's a template brace
+    and fails as CEL: write `record.x`, not `{record.x}`.
+
 ---
 
 ## CRM Automation Blueprint