Context Builder MVP: core selection and assembly model

[jamesrochabrun]

Parent: #398

Goal

Add the non-UI spine for curated launch context: selected project-relative paths, bounded file reads, assembled XML-tagged context, and launch prompt composition.

Implementation Notes

Introduce a small context model and assembly layer before building richer UI.

Suggested types:

• ContextSelection: selected paths, optional slices placeholder, code-map path placeholder, include-git-diff flag
• ContextSection: file map, file contents, git diff, user instructions
• ContextAssembler: builds the exact prompt block from selection + file contents + user instructions
• ContextAccountant actor or equivalent: resolves selected file contents off-main with bounded concurrency

Launch integration target: CLISessionsViewModel.startNewSessionInHub. Add an optional contextBlock parameter and combine it with initialPrompt before routing to Claude terminal submission or Codex process launch.

For large assembled context, write a temp markdown file under AgentHub-owned temp/application-support space and pass a prompt reference to that file instead of pasting a huge block.

Acceptance Criteria

• Given three selected files, the assembler produces a deterministic block containing those files under structured tags.
• startNewSessionInHub can receive contextBlock and prepends it to the user prompt.
• Empty context preserves existing launch behavior.
• Oversized context uses a temp-file reference instead of inlining.
• File reads do not happen on @MainActor.

Tests

Add focused tests for assembly output, empty selection, missing/unreadable file behavior, and temp-file fallback.

Run targeted AgentHubCore tests for the new suite plus any existing launch tests touched.