phase 13: workspace/rename + executeCommand router + nuwiki.file.delete

Server-side rename with cross-document link rewriting (P11's
read_or_open delivers on its promise) and the first executeCommand
entry. Plus the routing infrastructure that Phases 14-17 will hang
their own commands off of.

commands.rs:
- commands::execute(backend, name, args) dispatches by command name
  and returns the WorkspaceEdit to apply.
- COMMANDS const slice is the source of truth for what's advertised
  in ExecuteCommandOptions.commands.
- First entry: nuwiki.file.delete with args { uri: <Url> } returning
  a WorkspaceEdit with a DeleteFile op.

rename.rs:
- compute_rename(backend, target_uri, new_name, utf8):
  1. Resolve the wiki via wiki_for_uri.
  2. Build new URI from wiki.root + new_name + extension.
  3. Snapshot the index's backlinks for the old page name before
     releasing the read lock so we don't hold it across awaits.
  4. For each backlink, read_or_open the source (live state or
     disk + reparse) and validate the cached span against current
     text.
  5. Rewrite the [[...]] segment with rewrite_wikilink_target,
     preserving anchor and description.
  6. Add the RenameFile op so the builder emits it before the text
     edits in documentChanges ordering.
- build_new_uri handles subdirectory paths and avoids double-
  appending the file extension.
- rewrite_wikilink_target is pub so other tooling can call it.

lib.rs:
- Capabilities: rename_provider + execute_command_provider listing
  commands::COMMANDS.
- New rename handler: cursor-on-wikilink renames the linked page,
  else the buffer's file. Delegates to rename::compute_rename.
- New execute_command handler dispatches via commands::execute, then
  pushes the resulting WorkspaceEdit through client.apply_edit. On
  unknown command or bad args, logs and returns null.
- read_or_open loses its allow(dead_code) attribute now that Phase
  13 calls it.
- rename_target_uri helper shares index-resolution with goto_def.

Tests (11 new in phase13_rename_commands.rs):
- rewrite_wikilink_target: plain, with description, with anchor,
  with both, subdirectory path, non-wikilink-text rejection.
- build_new_uri: under root, with subdirectory, extension dedup,
  empty path segments skipped.
- nuwiki.file.delete builder smoke test (DocumentChanges + DeleteFile).

All 211 v1.0+Phase 11+12 tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/src/commands.rs

@@ -0,0 +1,51 @@
+//! `workspace/executeCommand` dispatcher.
+//!
+//! Phase 13 lands the router + the first command. Phases 14–17 register
+//! their own commands here. Each handler returns the `WorkspaceEdit` it
+//! wants applied; the `Backend` calls `client.apply_edit` after the call.
+//!
+//! Phase 13 surface:
+//!   - `nuwiki.file.delete` — args: `{ "uri": <Url> }`
+
+use serde::Deserialize;
+use tower_lsp::lsp_types::{Url, WorkspaceEdit};
+
+use crate::edits::{op_delete, WorkspaceEditBuilder};
+use crate::Backend;
+
+/// All commands the server advertises in `ExecuteCommandOptions.commands`.
+/// Keep this in lock-step with the match arms in `execute` below — adding
+/// here without a handler will fail at runtime; adding a handler without
+/// listing here means the client won't see it as available.
+pub const COMMANDS: &[&str] = &["nuwiki.file.delete"];
+
+/// Dispatch an `executeCommand` request. Returns `Ok(None)` for commands
+/// that don't produce a `WorkspaceEdit` (e.g. read-only queries); returns
+/// `Err` with a human-readable message for unknown commands or bad args
+/// — the LSP backend logs it and returns `null` to the client.
+pub(crate) async fn execute(
+    _backend: &Backend,
+    command: &str,
+    args: Vec<serde_json::Value>,
+) -> Result<Option<WorkspaceEdit>, String> {
+    match command {
+        "nuwiki.file.delete" => file_delete(args),
+        other => Err(format!("unknown nuwiki command: {other}")),
+    }
+}
+
+fn file_delete(args: Vec<serde_json::Value>) -> Result<Option<WorkspaceEdit>, String> {
+    #[derive(Deserialize)]
+    struct Args {
+        uri: Url,
+    }
+    let raw = args
+        .into_iter()
+        .next()
+        .ok_or_else(|| "nuwiki.file.delete: missing arguments".to_string())?;
+    let parsed: Args = serde_json::from_value(raw)
+        .map_err(|e| format!("nuwiki.file.delete: invalid args — {e}"))?;
+    let mut builder = WorkspaceEditBuilder::new();
+    builder.file_op(op_delete(parsed.uri));
+    Ok(Some(builder.build()))
+}