docs: add parameter detector design doc

Client-side PowerShell parameter detection with stepper UI for
converting hardcoded values to template parameters.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/plans/2026-03-14-parameter-detector-design.md

@@ -0,0 +1,116 @@
+# Parameter Detector — Design Doc
+
+> **Date:** 2026-03-14
+> **Status:** Approved
+> **Scope:** Frontend only (no backend changes)
+
+## Overview
+
+A client-side tool in the Script Template Editor that scans a PowerShell script body, detects hardcoded values that should be parameterized, and walks the user through converting them one-by-one via a stepper UI.
+
+## Detection Engine
+
+Pure TypeScript utility (`lib/scriptParameterDetector.ts`) that takes a script body string and returns `ParameterCandidate[]`.
+
+### Detection targets (in order)
+
+1. **Script-level `param()` blocks** — the `param(...)` block at the top of the script, before any `function` keyword. Extracts name, type annotation, and default value. Skips `param()` blocks inside `function` declarations.
+2. **Variable assignments** — `$VarName = 'value'`, `$VarName = "value"`, `$VarName = 123`, `$VarName = $true/$false`. Skips variables already found in the param block and PowerShell internals like `$ErrorActionPreference`.
+
+### Type inference
+
+| Detection | Suggested Type | Sensitive |
+|-----------|---------------|-----------|
+| `[string]` or plain string value | `text` | no |
+| `[switch]` or `$true`/`$false` | `boolean` | no |
+| `[int]`, `[int32]`, `[int64]` or numeric value | `number` | no |
+| `[SecureString]` or name contains password/secret/key/credential | `password` | yes |
+| No type info, string value | `text` | no |
+
+### Candidate shape
+
+```typescript
+interface ParameterCandidate {
+  variableName: string        // "$OUPath"
+  suggestedKey: string        // "ou_path"
+  suggestedLabel: string      // "OU Path"
+  suggestedType: ScriptParameter['type']
+  sensitive: boolean
+  defaultValue: string | boolean | number | null
+  source: 'param_block' | 'assignment'
+  lineNumber: number
+  matchedLine: string
+  inferenceReason: string     // "Detected [switch] type declaration"
+}
+```
+
+## Stepper UI
+
+`ParameterDetectorStepper` component renders inline below the ScriptBodyEditor in the Script Body section.
+
+### Trigger
+
+- "Detect Parameters" button (secondary style, Wand2/Scan icon) below the script body textarea
+- Hidden if script body is empty; disabled while stepper is active
+- If no candidates found: brief "No parameter candidates detected" message
+
+### Stepper layout
+
+Shows one candidate at a time with:
+- Progress indicator ("Candidate 2 of 5" + dots)
+- Matched line displayed in monospace
+- Editable fields: Key, Label, Type (with info icon showing inferenceReason), Default value
+- Checkboxes: Required, Sensitive
+- Actions: Skip, Accept & Next (last item: Accept & Finish / Skip & Finish)
+
+### On accept
+
+1. Script body: replace the hardcoded value with `{{key}}`
+2. Parameters schema: append a new `ScriptParameter` with suggested values + original value as `default`
+
+### Edge cases
+
+- Script body edited during detection → stepper dismisses
+- Key conflicts with existing parameter → warning + suggested alternative
+- Re-running after partial conversion → skips already-converted `{{key}}` placeholders
+
+## Integration
+
+### Component tree
+
+```
+ScriptTemplateEditor
+├── Metadata section
+├── Script Body section
+│   ├── ScriptBodyEditor
+│   ├── "Detect Parameters" button          ← NEW
+│   └── ParameterDetectorStepper            ← NEW (conditional)
+├── Parameters section
+│   └── ParameterSchemaBuilder
+└── Fixed Action Bar
+```
+
+### Data flow
+
+- Detection runs client-side via `detectParameterCandidates(script_body)`
+- Candidates stored in local React state on ScriptTemplateEditor
+- Accept updates `form.script_body` and `form.parameters_schema` via existing `updateField()`
+- `isDirty` flag set automatically — user can cancel without saving to undo everything
+- No new backend endpoints needed
+
+## File changes
+
+**New files:**
+- `frontend/src/lib/scriptParameterDetector.ts`
+- `frontend/src/components/script-editor/ParameterDetectorStepper.tsx`
+
+**Modified files:**
+- `frontend/src/components/script-editor/ScriptTemplateEditor.tsx`
+- `frontend/src/types/scripts.ts`
+
+## Out of scope
+
+- No backend changes
+- No AI-powered detection (future enhancement)
+- No auto-detection on paste
+- No individual undo for accepted parameters