feat: add conditional execution for tasks and commands (#2564)

website/src/docs/reference/schema.md

@@ -616,6 +616,27 @@ tasks:
       - ./deploy.sh
 ```
 
+#### `if`
+
+- **Type**: `string`
+- **Description**: Shell command to conditionally execute the task. If the
+  command exits with a non-zero code, the task is skipped (not failed).
+
+```yaml
+tasks:
+  # Task only runs in CI environment
+  deploy:
+    if: '[ "$CI" = "true" ]'
+    cmds:
+      - ./deploy.sh
+
+  # Using Go template expressions
+  build-prod:
+    if: '{{eq .ENV "production"}}'
+    cmds:
+      - go build -ldflags="-s -w" ./...
+```
+
 ### `dir`
 
 - **Type**: `string`
@@ -812,6 +833,27 @@ tasks:
           SERVICE: '{{.ITEM}}'
 ```
 
+### Conditional Commands
+
+Use `if` to conditionally execute a command. If the shell command exits with a
+non-zero code, the command is skipped.
+
+```yaml
+tasks:
+  build:
+    cmds:
+      # Only run in production
+      - cmd: echo "Optimizing for production"
+        if: '[ "$ENV" = "production" ]'
+      # Using Go templates
+      - cmd: echo "Feature enabled"
+        if: '{{eq .ENABLE_FEATURE "true"}}'
+      # Inside for loops (evaluated per iteration)
+      - for: [a, b, c]
+        cmd: echo "processing {{.ITEM}}"
+        if: '[ "{{.ITEM}}" != "b" ]'
+```
+
 ## Shell Options
 
 ### Set Options