refactor: tangent - migrate tangetproectcontext onto mobx

[maxy-shpfy]

Description

Extracts the Tangent project shell's UI state out of TangentProjectContext and into a new MobX-based TangentProjectStore class. Previously, all session, chat tab, and workarea tab logic lived as a large collection of React state, refs, and functions inside the context provider. The store now owns this state as observable properties, making it the single source of truth for which session is active and each session's chat and workarea tabs.

Key changes:

• TangentProjectStore replaces the inline context value, exposing observable computed properties (activeSessionId, workareaTabs, chatTabs, etc.) and actions (selectSession, openAgent, closeChatTab, openResolvedView, closeWorkareaTab, etc.)
• Components that previously destructured many values from useTangentProject() now access the store directly and are wrapped with MobX observer so they re-render reactively
• Server data (project details, resources, sessions list) is moved out of the context entirely and fetched directly in the components that need it (ResourcesWindowContent, ProjectHeader, SessionsWindowContent)
• AttachResourceInput, ProjectResourceKind, and ProjectResourceItem types are relocated to the files where they are used rather than being exported from the context
• useTangentSessionTabs hook and its test file are deleted; the equivalent logic now lives in TangentProjectStore
• TangentProjectAgentProvider no longer maintains a depsRef and a getActiveTabBridgeRef; instead it reads directly from the store instance, which MobX keeps current
• onError callbacks in components are replaced with direct calls to useToastNotification
• A comprehensive test suite for TangentProjectStore is added covering chat tabs, workarea tabs, bridge resolution, and environment waiting

Screenshots (if applicable)

Test Instructions

1. Open a Tangent project and verify sessions can be started and switched between, with each session's chat and workarea tabs restored correctly on switch.
2. Open pipeline resources from the resources dock and confirm they open in the workarea and that removing a resource works.
3. Send a chat message and confirm the session is not auto-discarded on switch; switch away from a never-prompted session and confirm it is discarded.
4. Open an agent sub-tab, close it, and verify fallback to the Chat tab.
5. Run the new TangentProjectStore.test.ts suite to confirm all unit tests pass.

Additional Comments

Server-fetched data (project, resources, sessions) intentionally remains in TanStack Query and is not moved into the store. The store owns only UI state that multiple parts of the shell and the out-of-render agent tools need to share.

[github-actions]

🎩 Preview

A preview build has been created at: 09-16-refactor_tangent_-_migrate_tangetproectcontext_onto_mobx/17587d6

[maxy-shpfy]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: create a project from a tile in the grid #2735 : 3 dependent PRs (#2736 , #2737 , #2738 )
• feat: projects dashboard #2734
• chore: tangent - annotate runs submission to projectid #2758
• fix: tangent - proper debug sequence system prompt #2757
• fix: tangent - quick run button from mini content window #2744
• fix: tangent - remote agent send delta immediately #2743
• fix: tangent - mint proper authorization with tangent #2742
• feat: tangent - debug in tangent sequence #2741
• chore: add autolayout tool #2740
• feat: tangent - resource management via document entity #2733
• feat: tangent - interactive chips in tangent chat #2732
• chore: save id stack to replay ids on reloads #2731
• refactor: tangent - migrate tangetproectcontext onto mobx #2730 👈 (View in Graphite)
• feat: tangent - keep track of open tabs while switching sessions inside project #2729
• feat: tangent - remote agent for runview and editor #2728
• refactor: tangent - structured workarea target #2727
• feat: tangent - embedded pipeline run view #2726
• feat: tangent - embedded pipeline editor #2725
• feat: tangent - dynamic workarea remote tools #2724
• feat: tangent - resources management in project view #2723
• feat: tangent project view page #2722
• feat: workspaces and projects service #2720
• chore: api client for workspaces and projects #2719
• master

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