Add specs/schema.md

specs/schema.md

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+# Flow Spec Schema
+
+The YAML spec format for declarative ST AR flow definitions.
+
+## Top-Level Structure
+
+```yaml
+version: "1.0"          # Spec format version
+meta:                   # Human-readable metadata
+  name: <string>
+  description: <string>
+application: ...        # AR Application definition
+subscription: ...       # Subscription definition
+routes: ...             # Route definitions (simple, template, composite)
+prerequisites: ...      # Resources that must exist before deploy
+```
+
+## application
+
+```yaml
+application:
+  name: <string>        # AR application name (must be unique in ST)
+  type: AdvancedRouting # Always AdvancedRouting for AR flows
+```
+
+## subscription
+
+```yaml
+subscription:
+  account: <string>         # ST account name that owns the subscription
+  folder: /<path>           # Upload folder that triggers routing (absolute path)
+  application: <string>     # AR application name (must match application.name)
+  post_transmission_actions:
+    ptaOnSuccessDoInAdvancedRoutingWildcardPull: true
+    submitFilenamePatternExpression: "*"   # "*" = trigger on any file
+    submitFilterType: FILENAME_PATTERN
+    triggerFileOption: fail
+    triggerOnSuccessfulWildcardPull: true
+```
+
+Note: `submitFilenamePatternExpression` here only affects server-initiated pulls.
+For client-uploaded files, use route-level `condition` to filter by file type.
+
+## routes.simple — Steps
+
+Steps are applied in array order. Each step sees all files in the route payload
+unless `usePrecedingStepFiles: true` is set.
+
+### Compress step
+
+```yaml
+- type: Compress
+  conditionType: ALWAYS           # Required. ALWAYS or EL.
+  actionOnStepFailure: PROCEED    # PROCEED = non-matching files continue
+  fileFilterExpression: "*.txt"   # GLOB or REGEX
+  fileFilterExpressionType: GLOB
+  usePrecedingStepFiles: false    # false = see all route files
+  singleArchiveEnabled: false     # false = one ZIP per file; true = bundle all
+  compressionMode: ZIP            # ZIP (default) or GZIP
+```
+
+Output filename: `<original>.zip`
+
+### PgpEncryption step
+
+```yaml
+- type: PgpEncryption
+  conditionType: ALWAYS
+  actionOnStepFailure: PROCEED
+  fileFilterExpression: "*.md"
+  fileFilterExpressionType: GLOB
+  usePrecedingStepFiles: false
+  encryptKeyExpression: <cert-alias>          # The cert name in ST cert store
+  encryptKeyExpressionType: ALIAS             # ALIAS or EXPRESSION_WILDCARD only
+  encryptKeyOwnerExpression: <account-name>   # Account that owns the cert
+  encryptKeyOwnerExpressionType: NAME
+  compressionType: "0"                        # "0" = no PGP compression
+  useAsciiArmour: false                       # false = binary; true = ASCII armored
+  signKeyExpression: null                     # Optional: signing key alias
+  signKeyExpressionType: null
+  signKeyOwnerExpression: null
+  signKeyOwnerExpressionType: null
+```
+
+Output filename: `<original>.pgp`
+
+**Cert requirements:** `accessLevel: PUBLIC`, `account: <owner-account>` (not null).
+`encryptKeyExpressionType` only accepts `ALIAS` or `EXPRESSION_WILDCARD` — not `NAME`.
+
+### SendToPartner step
+
+```yaml
+- type: SendToPartner
+  conditionType: ALWAYS
+  actionOnStepFailure: FAIL
+  fileFilterExpression: "*"
+  fileFilterExpressionType: GLOB
+  usePrecedingStepFiles: false
+  partnerName: <partner-account>              # ST partner account name
+  partnerSite: <partner-site-name>            # ST partner site name
+```
+
+### ExternalScript step
+
+```yaml
+- type: ExternalScript
+  conditionType: ALWAYS
+  actionOnStepFailure: PROCEED
+  fileFilterExpression: "*"
+  fileFilterExpressionType: GLOB
+  usePrecedingStepFiles: false
+  scriptPath: /path/to/script.sh
+  outputFileExpression: "${basename}.out"
+```
+
+## routes.template
+
+```yaml
+template:
+  name: <string>
+  conditionType: MATCH_ALL    # Always MATCH_ALL for templates
+```
+
+## routes.composite
+
+```yaml
+composite:
+  name: <string>
+  conditionType: MATCH_ALL    # Always MATCH_ALL for composites
+```
+
+## prerequisites
+
+Resources validated by `st_env_snapshot.py` before any deploy:
+
+```yaml
+prerequisites:
+  accounts:
+    - name: <account-name>
+      type: individual | partner
+  partner_accounts:
+    - name: <account-name>
+  partner_sites:
+    - name: <site-name>
+      partner: <partner-account-name>
+  certificates:
+    - name: <cert-alias>
+      type: pgp | ssh | x509
+      usage: partner | local | private
+      account: <owner-account>
+      accessLevel: PUBLIC | PRIVATE
+```
+
+If any prerequisite is missing, `st_deploy.py` exits with code 2 before making any changes.
+
+## EL Conditions (route-level filtering)
+
+To filter which files enter a route by extension:
+
+```yaml
+routes:
+  simple:
+    conditionType: EL
+    condition: "${transfer.target.matches('.*\\.txt')}"
+```
+
+- `transfer.target` = full file path
+- Use Java regex with double-escaped dot (`\\.` not `\.`)
+- Single-escape throws AR0045 at runtime