docs: restore animated README demo

[NikolayS]

Summary

• restore the animated README GIF embed after regenerating it from the current 2.0 reader flow
• update the demo container to install from sql/ash-install.sql instead of the removed devel/sql/ash-install.sql
• fix the recorder post-process so the first GIF frame is the visible pg_ash banner, not a blank terminal
• update demo docs to describe the GIF as the reviewed README asset again

Manual Review Notes

The regenerated GIF is a real recording against a live Postgres 18 demo container. It shows:

• ash.status() with version=2.0-beta1, sampling enabled, and pg_cron available
• ash.periods() AAS triage
• colored ash.chart(...)
• ash.top('wait_event', ...)
• ash.top('query_id', wait_event => 'Lock:tuple', ...)
• reverse drilldown with ash.top('wait_event', query_id => :top_qid, ...)

Generated asset:

• demos/ash_demo.gif: 1020x462, 843 frames, 380K
• demos/ash_demo.cast: 52K

Validation

• make -C demos record completed successfully
• gifsicle --unoptimize --explode demos/ash_demo.gif produced 843 full-size frames
• manually inspected frames 0, 250, 500, and 842
• frame 0 now shows the green pg_ash banner, not a blank terminal
• git diff --check
• bash -n demos/record.sh demos/container-entrypoint.sh demos/workload.sh
• cast scan clean for ERROR, syntax error, No such file, does not exist, FATAL, PANIC, Traceback, command not found
• local docs-lint guard from .github/workflows/test.yml: docs-lint-guard-ok

Merge Gate

Draft PR only. Do not merge until CI is green, REV/samorev passes and is posted, README/GIF is manually reviewed, and human explicit approval is given.

[NikolayS]

Closing as superseded by #140.

#140 contains the same installer-path repair with a newer end-to-end recording, and adds the reliability work this branch lacks: installer discovery through ash_sql_chain.py, a setup-readiness handshake, bounded failure diagnostics, positive and negative Docker smoke gates, shell hardening, and workload signal/exit-status preservation. Its corrected head passed all hosted checks and the refreshed artifacts were inspected at original resolution.

The branches diverged from the same base and both replace demos/ash_demo.cast and the binary demos/ash_demo.gif while editing the demo entrypoint and recorder. A trial merge produces text conflicts and a binary GIF conflict. This PR also edits frozen sql/ash-install.sql; its docs-lint check fails on that release-freeze guard.

The valuable README-only idea here should not be lost. After #140 lands, a focused follow-up can embed #140’s reviewed demos/ash_demo.gif and update the demo wording without regenerating assets or touching sql/. The optional pg_cron explanation and remaining sentence-case headings can be evaluated as separate README-only edits.

Superseded by #140. Thank you for the recording and visual-review groundwork.