Cleanup/remove dead files empty folders

[fantauzza]

Summary

Cleaning up gitignore and unused/unuseful/way-stale files that could lead developers astray or waste their time.

Cleanup pass that removes dead/abandoned/state files and the stale .gitignore rules that hid them, plus a few rules that no longer matched anything. One intentional addition: the percentile-band charts are now tracked as a reference. Rationale table for the gitignore diff is below. No code changes.

If there are any other files that you have questions about or are getting in your way please let me know and I’ll take a look.

Files deleted from the repo that were tracked
database/reference/ - orphaned reference snapshots from February. Outdated clutter, nothing references them.
database/register_developer.sql - old one-off INSERT into columns that no longer exist. Wouldn't run today.

Local files deleted (never tracked - explains the gitignore rule changes below)
These were gitignored, so they were never in the repo. Deleting them just lets us drop the matching ignore rule.

database/secrets/ -a single orphaned AWS key from early presign-download work. The download path no longer uses it.
database/setup_db_connection.sh - unused connection helper, superseded by the per-user env setup (~/.coeqwal-env- + become) documented in database/README.md.
database/setup_developer_session.sql - dead bootstrap script that errors on run. Current onboarding is documented in database/README.md ("Setting up a new developer").
database/scripts/*.sh - old scripts from an old process that built tables from seed data from the S3 bucket. Seed tables now live in the repo and that bucket is gone.
etl/reference/ - local stash of DSS files used to test the ETL during development. Personal scratch, not repo content.
costs/ - local scratch from AWS cost/resource exports run in the console

Archive stashes (scripts/.archive/, api/coeqwal-api/routes/_archive/) - abandoned prototype modules (network-visualization experiments, old development backup files).

Content added
etl/statistics/charts/ is now tracked (removed from .gitignore). Helpful reference for how percentile-band charts work and how they show lay audiences the probability of their deliveries/storage/flow.

.gitignore rules removed
Rules whose files were deleted above:

**/.archive/, **/_archive/ - local archive stashes removed
database/secrets/ - this was gitignored, so never committed to the repo (no git history to worry about). It held one stale local AWS key from early presign-download work that nothing uses anymore
database/setup_db_connection.sh, database/setup_developer_session.sql, database/scripts/*.sh - dead helpers deleted.
etl/reference/ - local DSS stash removed.
costs/ - local cost dumps removed.

Dead/redundant rules that matched nothing:

database/scripts/sql/**/*_local.sql, *_cloud9.sql - no files follow this pattern anymore
/docs/INFRASTRUCTURE.md - now lives in the team-accessible private repo.
audits/ cleanup: dropped !/audits/README.md (no such file, probably consolidated into another file in last PR), /audits/.tar.gz (redundant - already covered by /audits/, compressed files were manually generated by me to transfer the audits around), and /audits/monthly_/.tar.gz (no tarballs are written inside monthly_* dirs). No behavior changed: ignore everything under audits/, keep the committed monthly_* snapshots.