Document the `~/.sentrysearch/` cache layout in the README

[ssrajadh]

Problem

SentrySearch writes several files under ~/.sentrysearch/ but none of them are documented. A user who wants to back up, clear, or migrate state has to read the source.

Real files written there:

File	Written by
db/	sentrysearch/store.py:11 (ChromaDB)
.env	sentrysearch/cli.py:11 (sentrysearch init)
dlq.json	sentrysearch/dlq.py:12 (failed-chunk dead-letter queue)
last_clip.json	sentrysearch/_toolkit_cache.py:19 (consumed by SentryBlur `--last`)
last_search.json	sentrysearch/_toolkit_cache.py:20 (consumed by SentryMerge `--last`)
history	sentrysearch/cli.py:1021 (readline history for `sentrysearch shell`)

Suggested fix

Add a "Cache & state" subsection to the README (probably under "Managing the index"). For each file, document:

• What writes it
• What it's used for
• Whether it's safe to delete (and what happens if you do)

Acceptance

A new contributor can answer "what is ~/.sentrysearch/history?" from the README alone.

Scope

Pure docs — only README.md changes.