docs: salvage the canon PR #140 wrote into the skills

[prorochestvo]

#140 was opened this morning against 16ae37c and slims CLAUDE.md by moving depth into the
three project skills. It conflicts now — in CLAUDE.md and beacon-storage — and the
conflicts are mine: it merged clean against alpha before today's outlook, #139 and #141
landed on the same files. That was a preflight I did not run.

Much of what it moved has since arrived by other routes: the outlook work carried its own
skill sections, and #141 relocated the SQLite PRAGMA and write-lock depth. What did not
arrive is canon written nowhere else. Taken here verbatim rather than paraphrased, because it
is someone else's writing and it is better than a restatement would be.

Salvaged

beacon-collection — the source kind and the options JSON column; the cmd/doctor
operator umbrella; and the reason the Telegram bot bypasses the proxy in code rather than by
configuration: it is the channel that reports collection failures, so it must not share a
failure mode with the thing it reports on. That "why" existed nowhere.

beacon-http-api — the HMAC scheme, spelling out that the literal "WebAppData" is the
key and the bot token is the message, which is the way round that is easy to invert; the
--api-dsn rationale (it travels with the deployment, not with the secrets file); and the
whole PublicError contract with the three assertions every controller error test owes.

beacon-storage — the repository pattern, the source-deletion cascade, and deploy-time
schema reconciliation.

All four skill description blocks are replaced by #140's, whose load triggers are strictly
wider — proxyutil, domain.RateSourceOptions, BEACON_CHROMIUM_PATH,
internal/tools/tgwebapp/initdata.go, internal.PublicError, RequireMigratedSchema,
stubSQLiteDBThrough and more. A skill that does not load is knowledge that is lost, so this
is the half of #140 with the longest reach.

CLAUDE.md

Thinned only where that depth moved, 19330 → 17823. Every tripwire keeps its sentence:
the inner-mux bypass, the batching, the cascade warning, the three-assertion rule.

Not taken: #140's restructuring of CLAUDE.md into ## Architecture / ## Tripwires /
## Configuration (12054 chars). It is a larger editorial change than a salvage, the file
is already 2177 under its budget, and it deserves to be its own decision rather than a side
effect. The branch stays, so it can be revisited.

Nothing dropped

backticked spans in alpha's CLAUDE.md: 210    absent from the new set: 0
distinct figures:                       15    absent:                  0

scripts/lint-checks.sh and go vet clean. Docs only — no Go changed.

Refs #140