Use Claude CLI model aliases instead of pinned model ids

[Th1nkK1D]

Why

The Claude backend pins exact model ids (claude-opus-4-8, claude-sonnet-4-6, claude-haiku-4-5), so we can't use the newest model considered how fast it changes. Moreover, normalizeClaudeModel silently replaces any id outside that list with Sonnet 4.6, so users can't work around it by editing their config.

How

• Use the Claude Code CLI's model aliases (opus / sonnet / haiku) as the model ids. It will resolves to the latest model in that family, so the picker stays current without a Codiff release.
• Sonnet stays the default.
• Existing configs keep working: normalizeClaudeModel maps any legacy claude-(opus|sonnet|haiku)… id to its alias before validating.
• Updated config/defaults.json, the config JSON schema (default + description), and the README example. The schema keeps claudeModel as a free string to support legacy values.

Related issue: #177 fixed the same "unknown ids silently replaced" problem for Codex by reading the model catalog from the Codex app-server. Unfortunately, Claude CLI has no equivalent listing, so aliases should be the way to go.

Verification

• vp check --fix: clean
• vp test: new/updated cases in electron/__tests__/claude.test.ts pass (legacy ids → aliases, unknown → sonnet)
• vp run build: passes
• Test Claude CLI alias model resolving with claude -p --model sonnet --output-format json
• Manual testing with legacy name, and after changing model to the new alias from the options.

AI disclosure

Research, code, tests, and this PR description were drafted with Claude Code (Claude Opus). I reviewed the plan, references, code diff, manual tested to confirm by myself, and updated the PR details. Any AI-assisted replies to review comments will be noted.