feat(channels): suppress_placeholder toggle for Discord and Slack

[tarrencev]

Summary

• Adds a suppress_placeholder boolean to DiscordConfig and SlackConfig (also wired through each channel's DB-instance factory).
• When enabled on Discord, the "Thinking..." placeholder + later edit is skipped; the typing indicator runs for the full agent turn (TTL raised 60s → 30 min as a safety net) and the final response is posted as a new message.
• When enabled on Slack, the placeholder is skipped; streaming is force-disabled (Slack streaming edits the placeholder) and the thinking_face reaction (when reaction_level is minimal/full) acts as the in-progress indicator.
• Extracted placeholderSuppressed() helper on the Slack channel to replace 5 repeated pointer-deref checks.

Why

On channels with a proper presence API, the intermediate "Thinking..." text message + message-edit cycle can feel clunky, especially in shared groups or when users prefer a quieter indicator. WhatsApp and Telegram already skip this pattern; this PR brings the same option to Discord and Slack behind an opt-in flag.

Compatibility

• Default is unchanged (placeholder + edit); existing deployments see no behavior change.
• Retry notifications and non-streaming tool-status updates are delivered by editing the placeholder, so both are silently dropped when suppressed. That's the point of the toggle (minimize in-progress chatter) and is documented in docs/05-channels-messaging.md.

Pre-Landing Review

Ran the full review pipeline (4 specialists + red team on a 266-line diff). Findings:

• Testing specialist (4 informational) — flagged missing handler-level tests for the suppress paths. Skipped this round because the existing TestStreamEnabledSuppressPlaceholder covers the main risk vector; adding handler tests requires a session/postMessage spy harness that doesn't yet exist in the repo, which is larger than this PR's scope.
• Maintainability specialist (4 informational) — magic number, DRY violation (inverted-nil guard repeated 5 times), factory-duplication, struct tag alignment. Auto-fixed: named the Discord typing TTL constants and the helper. Skipped: factory.go DRY refactor (pre-existing) and struct alignment (gofmt wants to reflow the unrelated TelegramConfig struct — not touching upstream code).
• Security specialist — NO FINDINGS.
• Performance specialist — NO FINDINGS.
• Red Team (4 critical, 4 informational) — flagged the silent-drop of retry/tool-status notifications (intentional per feature design, now called out in docs), the 10-min TTL ceiling for long agent runs (fixed — bumped to 30 min with a named constant), concurrent-typing race (pre-existing, skipped), and a doc undersell (fixed).

PR Quality Score: 8.5/10

Test Plan

• go build ./... + go build -tags sqliteonly ./...
• go vet ./...
• go test ./internal/channels/... ./internal/config/... — all pass
• New TestStreamEnabledSuppressPlaceholder covers the Slack stream-disable interaction
• Manual Discord DM + group turn with suppress_placeholder: true — confirm typing runs for full turn, no "Thinking..." message, final response arrives as a new message
• Manual Slack DM + thread turn with suppress_placeholder: true + reaction_level: minimal — confirm no placeholder, thinking-face reaction fires, final response as a new message

Note: one unrelated pre-existing flaky test in internal/tools/shell_abort_test.go was noticed during /ship (process-group cleanup detecting orphan sleep 60 PIDs in the environment). No code in this PR touches internal/tools/.

🤖 Generated with Claude Code