docs: fix ADR turn-boundary-batching compliance rule precision

[obrutjack]

Summary

PR #598 (merged) introduced docs/adr/turn-boundary-batching.md. Two precision issues were identified during review but not resolved before merge.

Issues to fix

1. §6.4 Rule 1 — adapter-layer scope clarification

Rule 1 lists "broker expanding Discord <@123> mentions to @username strings" as a prohibited transformation. However, resolve_mentions() in discord.rs performs adapter-level preprocessing (stripping bot trigger mention, replacing role mentions) before {prompt} enters the broker pipeline.

Fix: Add a caveat clarifying Rule 1 applies to the broker/dispatcher layer, not adapter-level preprocessing. Example:

Note: Adapter-level preprocessing that runs before {prompt} is constructed (e.g. resolve_mentions()) is not subject to this rule.

References: obrutjack review, masami-agent re-review

2. §6.4 Rule 8 — scope to retry-failed case only

Rule 8 says SendError must surface immediately, but §2.5 designs a retry-once path where the first SendError triggers eviction + re-send silently. Only a failed retry should emit ❌ + ⚠️ + Err.

Fix: Scope Rule 8 to the retry-failed case. Keep the residual-loss note as-is.

Reference: chaodu-agent changes requested

Additional nits (optional)

• §3.2: Mark slack_ts_to_iso8601 as "(proposed helper)"
• Metadata "Supersedes" field: reword self-reference
• §6.6 threshold formula: clarify units (count × tokens = overhead in tokens)

Context

• ADR: docs/adr/turn-boundary-batching.md
• Merged PR: docs: add ADR for turn-boundary message batching #598
• Implementation PR: feat(dispatch): turn-boundary batching dispatcher v2 #686
• Tracking RFC: RFC: Turn-boundary message batching #580

Closes no existing issue — this is a follow-up from PR #598 review.