fix: update tool names to canonical VS Code format in principal-software-engineer agent (#1198)

Replace deprecated/incorrect tool references with canonical VS Code built-in
tool names per https://code.visualstudio.com/docs/copilot/reference/copilot-vscode-features:

 search/changes (add correct prefix)
 vscode/extensions (add correct prefix)
 read/problems (add correct prefix)
 search/usages (add correct prefix)
 execute/testFailure (add correct prefix)
 vscode/VSCodeAPI (correct casing and prefix)
 search/textSearch (renamed tool)
 read/terminalLastCommand (moved to read/)
 read/terminalSelection (moved to read/)
 execute (renamed tool set)
 execute/createAndRunTask (renamed)
 newWorkspace (renamed)
- Remove deprecated tools with no equivalent: findTestFiles, githubRepo,
  openSimpleBrowser, runTests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

agents/principal-software-engineer.agent.md

@@ -1,7 +1,7 @@
 ---
 description: 'Provide principal-level software engineering guidance with focus on engineering excellence, technical leadership, and pragmatic implementation.'
 name: 'Principal software engineer'
-tools: ['changes', 'search/codebase', 'edit/editFiles', 'extensions', 'web/fetch', 'findTestFiles', 'githubRepo', 'new', 'openSimpleBrowser', 'problems', 'runCommands', 'runTasks', 'runTests', 'search', 'search/searchResults', 'runCommands/terminalLastCommand', 'runCommands/terminalSelection', 'testFailure', 'usages', 'vscodeAPI', 'github']
+tools: ['agent', 'browser', 'edit', 'execute', 'github/*', 'read', 'search', 'todo', 'vscode', 'web/fetch']
 ---
 # Principal software engineer mode instructions