Add extension normalization command

specs/004-extension-rename/data-model.md

@@ -0,0 +1,66 @@
+# Data Model – Extension Command
+
+## Entity: ExtensionRequest
+- **Fields**
+  - `WorkingDir string` — Absolute path resolved from CLI `--path` or current directory.
+  - `SourceExtensions []string` — Ordered list of unique, dot-prefixed source extensions (case-insensitive comparisons).
+  - `TargetExtension string` — Dot-prefixed extension applied verbatim during rename.
+  - `IncludeDirs bool` — Mirrors `--include-dirs` scope flag.
+  - `Recursive bool` — Mirrors `--recursive` traversal flag.
+  - `IncludeHidden bool` — True only when `--hidden` supplied.
+  - `ExtensionFilter []string` — Normalized extension filter from `--extensions`, applied before source matching.
+  - `DryRun bool` — Indicates preview-only execution (`--dry-run`).
+  - `AutoConfirm bool` — Captures `--yes` for non-interactive apply.
+  - `Timestamp time.Time` — Invocation timestamp for ledger correlation.
+- **Validation Rules**
+  - Require at least one source extension and one target extension (total args ≥ 2).
+  - All extensions MUST start with `.` and contain ≥1 alphanumeric character after the dot.
+  - Deduplicate source extensions case-insensitively; warn on duplicates/no-ops.
+  - Target extension MUST NOT be empty and MUST include leading dot.
+  - Reject empty string tokens after trimming whitespace.
+- **Relationships**
+  - Consumed by the extension rule engine to enumerate candidate files.
+  - Serialized into ledger metadata alongside `ExtensionSummary`.
+
+## Entity: ExtensionSummary
+- **Fields**
+  - `TotalCandidates int` — Number of filesystem entries examined post-scope filtering.
+  - `TotalChanged int` — Count of files scheduled for rename (target extension applied).
+  - `NoChange int` — Count of files already matching `TargetExtension`.
+  - `PerExtensionCounts map[string]int` — Matches per source extension (case-insensitive key).
+  - `Conflicts []Conflict` — Entries describing colliding target paths.
+  - `Warnings []string` — Validation and scope warnings (duplicates, no matches, hidden skipped).
+  - `PreviewEntries []PreviewEntry` — Ordered list of original/new paths with status `changed|no_change|skipped`.
+  - `LedgerMetadata map[string]any` — Snapshot persisted with ledger entry (sources, target, flags).
+- **Validation Rules**
+  - Conflicts list MUST be empty before allowing apply.
+  - `NoChange + TotalChanged` MUST equal count of entries included in preview.
+  - Preview entries MUST be deterministic (stable sort by original path).
+- **Relationships**
+  - Emitted to preview renderer for CLI output formatting.
+  - Persisted with ledger entry for undo operations and audits.
+
+## Entity: Conflict
+- **Fields**
+  - `OriginalPath string` — Existing file path causing the collision.
+  - `ProposedPath string` — Target path that clashes with another candidate.
+  - `Reason string` — Short code (e.g., `duplicate_target`, `existing_file`) describing conflict type.
+- **Validation Rules**
+  - `ProposedPath` MUST be unique across non-conflicting entries.
+  - Reasons limited to known enum for consistent messaging.
+- **Relationships**
+  - Referenced within `ExtensionSummary.Conflicts`.
+  - Propagated to CLI preview warnings.
+
+## Entity: PreviewEntry
+- **Fields**
+  - `OriginalPath string`
+  - `ProposedPath string`
+  - `Status string` — `changed`, `no_change`, or `skipped`.
+  - `SourceExtension string` — Detected source extension (normalized lowercase).
+- **Validation Rules**
+  - `Status` MUST be one of the defined constants.
+  - `ProposedPath` MUST equal `OriginalPath` when `Status == "no_change"`.
+- **Relationships**
+  - Aggregated into `ExtensionSummary.PreviewEntries`.
+  - Used by preview renderer and ledger writer.