docs: re-render session-layout.png for the ~/.tilth sessions location (#8 follow-up)

[samkeen]

Follow-up to #8, which moved per-user state out of the Tilth clone's sessions/ and into ~/.tilth/sessions/<id>/.

The leftover

docs/assets/session-layout.png is a flat PNG (no SVG/HTML source) that still draws the old harness-side tree rooted at:

HARNESS SIDE — ~/projects/tilth/sessions/<id>/

The prose and alt text that reference it (docs/deep-dives/session-layout.md, docs/getting-started/running-the-demo.md) were updated to ~/.tilth/sessions/<id>/ in the #8 docs pass, so the image is now the only place still showing the pre-#8 path.

Needed change

Re-render the diagram so the HARNESS SIDE root reads:

~/.tilth/sessions/<id>/
├── workspace/        (git worktree)
├── events.jsonl
├── summary.json
└── checkpoint.json

Everything else stays as-is: the TARGET REPO SIDE (~/projects/tilth-demo/.git/ with refs/heads/session/<id> and worktrees/<id>/) and the sage-green "git worktree binds these" arrow are unchanged. Follow docs/assets/IMAGE_STYLE.md (anchored to brain-hands-session.png) so the visual voice matches. The alt text already in session-layout.md describes the target end-state and can serve as the render spec.

Lower priority (same theme)

The resume/reset terminal-capture screenshots (e.g. docs/assets/resume-after-iter-cap.png) still show uv run tilth resume. That form still works (contributor path), so re-capturing with the now-preferred bare tilth resume is optional polish, not a correctness fix.

Broader docs-pass tracking lives in #38; eventual streamlined publish is #34.