Preserve Mattermost Codex progress previews

Mattermost runs could deliver the final Codex response correctly while losing draft progress updates during tool-heavy execution because shared dispatch suppressed callbacks whenever source delivery was suppressed.

Threaded a Mattermost-specific progress-callback opt-in through inbound dispatch only when draft tool-progress preview is enabled, preserving final delivery policy and non-Mattermost behavior.

• Improves Mattermost operator experience by restoring visible progress during long Codex tool and reasoning runs.
• Keeps final reply ownership unchanged, avoiding accidental source-owned final messages while allowing channel-owned draft progress.
• Limits blast radius with a Mattermost preview-mode gate instead of changing shared dispatch defaults.

• Added Mattermost config and inbound system-event regressions for the source-suppressed progress callback gate.
• Verified the draft stream still receives progress content while final delivery remains unchanged.
• PR validation covered focused Vitest runs, oxlint, tsdown build, diff check, and local autoreview.