fix(messages): extract text from legacy attachments/blocks when content is empty

[nulligence]

Problem

`slack-cli message list`/`message get` return an empty `content`/`text` field for any message whose body lives entirely in a legacy Slack `attachments` block (or top-level Block Kit `blocks`) rather than the plain `text` field. `parseRawMessage` in `internal/slack/messages.go` only ever read `raw["text"]`.

This is common traffic for exactly the channels you'd most want reliable visibility into: Grafana-style alerting bots and other integrations that post via legacy `attachments` (e.g. a "critical-alerts-prod" style channel) consistently came back with a stub or empty body, forcing a manual `curl` against `conversations.history` to recover the actual alert text every time. The same empty-content symptom also showed up on an unrelated bot/integration whose real content lived in `attachments[0].blocks[]` rather than the legacy attachment fields at all, confirming this isn't integration-specific — it's any bot that doesn't populate plain `text`.

Fix

`parseRawMessage` now falls back to extracting human-readable text when `text` is empty:

1. From legacy `attachments` — `title`, `pretext`, `text`/`fallback`, `fields[].title`/`.value`, `footer`, and (since some integrations attach Block Kit content instead of the legacy fields) any nested `attachments[].blocks[]`.
2. From top-level Block Kit `blocks` — `text.text`, `fields[].text`, and recursively through `elements` (context/rich-text blocks).

Messages that already have a plain `text` field are completely unaffected — the fallback only runs when `text` is empty, so normal user/bot messages behave exactly as before.

Added unit tests in `internal/slack/messages_test.go` covering: plain-text messages (unaffected), legacy-attachment-only messages, attachment-with-only-`fallback`, an attachment carrying nested `blocks` instead of legacy fields, and top-level-`blocks`-only messages.

Test plan

• `go build ./...`
• `go test ./...` (all packages pass, including new `internal/slack` tests)
• `gofmt` clean on touched files

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