feat(voyage)!: FLAG_SCHEMA keys trek* + arg-parser test cases [skip-docs]

- Rename FLAG_SCHEMA keys ultrabrief|ultraresearch|ultraplan|ultraexecute|ultrareview|ultracontinue -> trek* equivalents
- Update 26 literal key references in tests/lib/arg-parser.test.mjs
- Update parseArgs($ARGUMENTS, 'ultracontinue') -> 'trekcontinue' in commands/trekcontinue.md
- trekendsession.md audited: no parseArgs invocation, no FLAG_SCHEMA entry needed

Atomic per session-spec: schema + tests + consuming commands committed together to
avoid vacuous-pass risk.

Part of voyage-rebrand session 2 (W3.2 / Step 4).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/ultraplan-local/lib/parsers/arg-parser.mjs

@@ -5,33 +5,33 @@
 // produces an `unknown_flags` array but does not error — the caller decides.
 
 const FLAG_SCHEMA = {
-  ultrabrief: {
+  trekbrief: {
     boolean: ['--quick', '--fg'],
     valued: [],
     aliases: {},
   },
-  ultraresearch: {
+  trekresearch: {
     boolean: ['--quick', '--local', '--external', '--fg'],
     valued: ['--project'],
     aliases: {},
   },
-  ultraplan: {
+  trekplan: {
     boolean: ['--quick', '--fg'],
     valued: ['--project', '--brief', '--export', '--decompose'],
     multi: ['--research'],
     aliases: {},
   },
-  ultraexecute: {
+  trekexecute: {
     boolean: ['--resume', '--dry-run', '--validate', '--fg'],
     valued: ['--project', '--step', '--session'],
     aliases: {},
   },
-  ultrareview: {
+  trekreview: {
     boolean: ['--quick', '--fg', '--dry-run', '--validate'],
     valued: ['--project', '--since'],
     aliases: {},
   },
-  ultracontinue: {
+  trekcontinue: {
     boolean: ['--help', '--cleanup', '--confirm', '--dry-run'],
     valued: [],
     aliases: {},