chore: promote expbkmain to bkmain (Mattermost link badge + agent view fixes)

[tusharbhardwaj-bk]

Promotes expbkmain (fe0622e17) to bkmain. Everything here has been running on expbkt3.

What ships

Two independent pieces of work:

1. Mattermost link badge (#146)
Sessions bound to a Mattermost conversation now show the Mattermost mark in the experimental sidebar's bottom-right lane, beside the priority badge, owner avatar and provider icon. Adds mattermostThreadUrl to thread metadata, mirroring the existing linearIssueUrl tag end to end (contract, decider, both projectors, projection column via migration 1023, client reducer), gated on a new threadMattermostLink capability.

The bridge side is already merged and deployed (t3-linear-bridge#55, live on bkt3automations.dev.beknown.live at ff82942), so the badge lights up automatically on new Mattermost binds once this lands.

2. Agent view fixes (#145 and precursors)
fix(web): disable untruthful agent views, fix(web): isolate same-origin agent view lifecycles, and a test drain fix. Not my work — carried along by the promotion.

Migration note

Migration 1023 adds a nullable mattermost_thread_url column to projection_threads. It ran cleanly against expbkt3's live database (243 MB, real data) with no backfill and no rewrite — it is a single additive ALTER TABLE ... ADD COLUMN.

Verification

• All 15 checks green on feat(web): mark sessions linked to a Mattermost chat #146; verified on expbkt3 end to end: deployed-sha matched the merge commit, service active, migration row present in the live DB, badge code present in the served bundle.
• Bridge deploy verified independently: binary contains the new symbols, service healthy on production's port.

Deploy impact

Merging this restarts t3-bkmain.service and ends every in-flight session on bkt3. Requested explicitly.

Model: Claude Opus 5 (1M context), harness: T3 Code.

🤖 Generated with Claude Code

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	7.2 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.4 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	55.7 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	11	—	21	✅
Claude	Total thread wire	—	13.6 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	7.2 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.4 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	56.5 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	11	—	21	✅

Baseline: unavailable · PR result: fe0622e · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 110.4 KiB
• Claude decoded thread snapshot: 111.1 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.