feat: add app.restorePreviousSession toggle to disable startup session restore

[evandhoffman]

What

Adds a user-facing setting to stop cmux from automatically restoring the previous launch (windows, workspaces, working directories, and terminal content) on startup.

Previously the only way to disable this was the undocumented CMUX_DISABLE_SESSION_RESTORE=1 env var — awkward for a GUI app. Now there's app.restorePreviousSession (default true, preserving today's behavior). When false:

• The automatic startup restore is skipped; each launch opens a single fresh workspace seeded by app.defaultWorkspacePath.
• The explicit Restore Previous Launch command (File menu / Command Palette / shortcut) still works — it's intentionally not gated, so you can pull the last session back on demand.

How

The gate lives in SessionRestorePolicy.shouldAttemptRestore() (Sources/SessionPersistence.swift), next to the existing CMUX_DISABLE_SESSION_RESTORE and automated-test gates, so all startup-restore suppression stays in one place. A restorePreviousSessionEnabled(defaults:) reader returns true when the key is unset (historical behavior).

Surfaces wired (mirrors the app.defaultWorkspacePath pattern from #5/#12)

• AppCatalogSection.restorePreviousSession — DefaultsKey<Bool>
• SessionRestorePolicy gate + default-true UserDefaults reader
• cmux.json boolean mapping (CmuxSettingsJSONPathSupport)
• JSON schema (web/data/cmux.schema.json)
• Settings → App → Restore Previous Session toggle + settings-search entry
• generated cmux.json template
• localization en + ja (Resources/Localizable.xcstrings)
• docs/configuration.md

Tests

Four regression tests in SessionPersistenceTests (a wired target), all passing locally:

• default is true when unset; reader reflects stored value
• shouldAttemptRestore(restorePreviousSessionEnabled: false) → skips restore
• shouldAttemptRestore(restorePreviousSessionEnabled: true) → proceeds

Validation

• xcodebuild Debug build: SUCCEEDED
• 4 new tests: passed
• pbxproj test-wiring lint: ok
• Swift file-length budget: regenerated (AppSection.swift, SessionPersistence.swift, test file grew)
• Localization audit: new Settings title/subtitle have en + ja; schema description is English-only description (matches defaultWorkspacePath precedent — no descriptionKey); template key and docs are not translatable UI strings.

https://claude.ai/code/session_01CNYr3XgjVDLsPNnPQxarn4