feat(slack): add explicit bounded history access

[mats-i]

Summary

• add an explicit, one-shot Slack history tool scoped to the active workspace, channel, and thread
• require deterministic fail-closed user authorization and bound reads to 12 messages without pagination
• prevent raw history from leaking into persistence, hooks, traces, fallback providers, Relay, MoA, or automatic thread hydration
• document the consent and retention contract

Verification

• 750 passed, 4 skipped across the affected agent, gateway, Slack, persistence, fallback, and streaming suites
• Ruff clean
• git diff --check clean
• fresh-context adversarial review: ACCEPT

Notes

• prior Slack content is never injected automatically; only an explicit direct request authorizes one read for the current turn
• the final assistant answer remains normally visible and persistable

[alt-glitch]

This was generated by AI during triage.

Related: #54535 and #29489 are open Slack history implementations for #6345. This PR adds a stricter one-shot authorization and ephemeral-retention design; maintainers should choose the preferred scope rather than treat the implementations as duplicates.

[andrexibiza]

Campaign interlock

Related: #6345. Related: #79772. Collision candidate; retain contributor credit while adjudicating #6345.