docs: add a Quick start section to README.md

[ncrmro]

Closes #407

Adds the copy-paste path from clone to a running dashboard right after the intro: prerequisites (uv, tmux, Docker), make sync / make start, where the dashboard appears, make stop, and a pointer to docs/overview.md for the concepts. Matches the existing Makefile targets.

Panopticon task: 8643a7031187448eac14276fa82e2c74

🤖 Generated with Claude Code

https://claude.ai/code/session_01NV7Vhip7N5QPuSuwvUvVpw

[luce-unsup]

Requesting changes on docs-structure grounds. The content itself is accurate — I checked the Makefile and make sync/make start/make stop all exist with the descriptions given — but the placement and framing collide with what the README and docs already establish:

1. Two getting-started paths with confusable titles. This README already has an end-user path: Install → Quickstart (pipx install panopticon-app, then panopticon quickstart). This PR adds a second, near-identically-titled ## Quick start teaching the contributor path (make sync/make start) without saying it is for developing panopticon itself. A newcomer reading top-down now meets two contradicting flows, and the repo already documents the split explicitly: docs/dev.md is titled "Developing panopticon" and opens with "Just want to use panopticon? Start with the README." That division is worth preserving.

2. Lower-fidelity duplicate of docs/dev.md. The new section drops git and the claude CLI from prerequisites (needed once you actually create a task) and omits make build, which dev.md's stack-up flow calls out as required before spawning task containers — so a reader following only make sync/make start gets a dashboard but can hit a missing base image on their first task.

3. Duplicate link. The closing sentence re-links docs/overview.md, which the intro paragraph immediately above the new section already links.

Suggested shape — keep the copy-paste path from #407 but frame it and defer the details:

Working on panopticon

Set up a dev environment and bring the stack up with make sync, then make start (make stop tears it down). The full development loop — checks, base-image build, migrations — is in docs/dev.md; to use panopticon, see Install below.

That honors the issue, avoids the Quickstart title collision, and leaves prerequisites and make build to the doc that already documents them correctly.

[luce-unsup]

Approved — the reframe addresses all three points: the section is now unambiguously the contributor path ("Working on panopticon"), prerequisites and make build are deferred to docs/dev.md which documents them correctly, and the duplicate overview.md link is gone. I re-checked the wording against the Makefile: make sync / make start / make stop descriptions all hold, and the #install anchor resolves to the existing Install heading.

One optional tidy-up, non-blocking: the PR description still describes the first revision (prerequisites list, "where the dashboard appears", overview pointer) — refreshing it to match the final section would keep the record straight for future readers.