Add persistent scratchpads and multi-window session restoration

[jeanmonet]

Summary

• add durable, automatically saved untitled Markdown scratchpads
• make ⌘T create a scratchpad tab, ⌘N create a scratchpad window, and ⌘⇧S promote a scratchpad to a named Markdown file
• restore the complete independent-window and native-tab hierarchy across normal quits, including selected tabs, editor continuity, usable frames, and physical-display affinity
• integrate the durable session with native AppKit window restoration and preserve existing version-1 sessions through migration

Per-window macOS Space assignment is intentionally not claimed here: AppKit does not expose a supported API for restoring a window to a specific Space.

Upstream base

Rebased onto upstream v0.5.1 (app version 0.5.1, build 11) at cf97f69. git range-diff confirms that the scratchpad/session feature patch is unchanged from the previously reviewed branch; only its upstream base and release-note context moved forward.

Verification

• make release — passed on the feature branch; production app and CLI built and signatures validated
• make smoke — passed again on the current v0.5.1 combined stack (7e7082d); bundle, CLI, document inspection/comparison, and app-window launch checks passed
• make benchmark-matrix — passed all five startup cases on the current v0.5.1 combined stack; worst visible-window p95 was 440.371 ms against the 500 ms limit, and worst target-ready p95 was 1,145.982 ms against the 1,250 ms limit
• make check-version — passed at 0.5.1 (11)
• git diff --check — passed
• make test — unavailable in this environment because only Command Line Tools are installed and the XCTest module is missing; focused XCTest coverage was added for scratchpad/session behavior but could not be executed locally

The startup matrix ran on macOS 26.5.2, Apple M1 Pro, with three warm-ups and ten measured direct launches per case. This is a single-machine development run, not a broad performance claim.

Compatibility and safety

• Ordinary Markdown remains authoritative and byte preservation is tested where relevant.
• App-only launch-path work was measured with the repository's startup matrix.
• CLI output, errors, and exit status remain deterministic where relevant. The CLI is unchanged.
• Protocol or schema changes include compatibility tests and documentation. The local session format advances to version 2 and migrates version 1.
• Generated files, private fixtures, credentials, and raw traces are excluded.
• I have signed off each commit under the Developer Certificate of Origin.

Scratchpads are private ordinary Markdown files under Application Support, use atomic autosave, flush synchronously during normal termination, and are removed after successful promotion or explicit discard. Physical-display placement falls back to a usable on-screen frame if a display is unavailable.

Documentation

• User-facing behavior and release implications are documented, or this change needs no documentation.