Document the DLQ and `--retry-failed` in the README

[ssrajadh]

Problem

The dead-letter queue is fully implemented — --retry-failed on index (sentrysearch/cli.py:390-391), a sentrysearch dlq subcommand group, and runtime messages like `Failed after N attempt(s), recorded to DLQ` (sentrysearch/cli.py:164) — but the README never mentions any of it. A user who sees "recorded to DLQ" in their indexing output has no documented next step.

Suggested fix

Add a "Failed chunks" subsection to the README (probably under "Managing the index") explaining:

• What the DLQ is and when chunks land in it (transient API failures vs. permanent — see _is_permanent_failure at cli.py:114)
• How to inspect it: sentrysearch dlq list (and any other dlq subcommands)
• How to retry: sentrysearch index <dir> --retry-failed
• Where it's stored: ~/.sentrysearch/dlq.json

Acceptance

Someone who sees "recorded to DLQ" in their indexing output can find the recovery flow in the README.

Scope

Pure docs — only README.md changes. Verify the dlq subcommand list against sentrysearch dlq --help before writing.