docs(readme): highlight session management with a demo gif

[SequeI]

Linked Issue

Closes #

Summary

Add a "Run many agents at once, detach, and check back later" section covering nono run --detached, ps, attach, and detach, linking to the full Session Lifecycle docs. Includes a recorded terminal demo (assets/sessions-demo.gif) showing a detached Claude Code session being listed, attached to, interacted with, and cleanly detached and stopped.

Test Plan

Checklist

• An issue exists and is linked above
• All commits are signed-off, using DCO
• All new code follows the project's coding standards (CLAUDE.md) and is covered by tests
• Public-facing changes are paired with documentation updates

[github-actions]

PR Review Summary

Size

Metric	Value
Lines added	+27
Lines removed	-1
Total changed	28
Classification	Small (< 50 lines)

Affected crates

No crate source directories are directly affected.

Blast radius — Contained

This PR touches: documentation

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_{Updated automatically on each push to this PR.}

[nogent-nolabs-ai]

nogent code review

No blocking issues; 0 suggestions.

Findings: none flagged in scope.

_{Automated code + security review. CI already covers clippy, rustfmt, tests, cargo-audit and commit-lint.}

[connrg]

lgtm

[lukehinds]

I think the text is good, but I may be able to capture a more high res terminal if sounds ok to you @SequeI ?

[SequeI]

@lukehinds Feel free to push up a betteer one on this branch and we can merge if looks good, I still need to learn how you get really crisp gifs from the terminal