fix: preserve hidden Slack links in agent prompts

[akshaan]

Summary

• Preserve Slack links when serializing Slackbot messages into the session API payload.
• Extract rich-text links from raw Slack blocks as a fallback, including hidden links behind visible text like continue.
• Append a compact Links included in the Slack message section to the agent prompt so linked Slack threads are visible to the agent.
• Add regression coverage for hidden Slack message links and duplicate-link avoidance.

Root Cause

Slack messages with manually hyperlinked words can arrive with plain adapter text such as continue while the URL lives in Slack rich-text blocks or adapter link metadata. Slackbot was forwarding only the plain text when it was non-empty, so the agent had no way to know a linked thread existed.

Validation

• bun test test/session-api.test.ts
• pnpm --filter slackbotv2 check:types

Note: the clean PR worktree does not have Node dependencies installed; validation was run in the main checkout against the same patch before creating the PR branch.