feat: add system prompt prepend mode for projects

[crystalin]

Summary

• Adds a new system_prompt_mode column to the projects table ('replace' | 'prepend', default 'replace')
• In prepend mode, the project's system prompt blocks are placed before the original request blocks instead of replacing them entirely
• In replace mode (default), existing behavior is unchanged — the project prompt replaces the entire system field

Changes

Database

• New migration 023-add-system-prompt-mode.ts adds system_prompt_mode VARCHAR(10) NOT NULL DEFAULT 'replace' to the projects table

Shared Package

• Added SystemPromptMode type ('replace' | 'prepend') to credentials.ts
• Updated Project and UpdateProjectRequest interfaces with system_prompt_mode field
• Updated getProjectSystemPrompt() query to return the mode field
• Updated updateProject() to handle system_prompt_mode updates

Proxy Service

• Updated applySystemPromptOverride() to support prepend mode
• Added normalizeSystemToBlocks() helper to convert string system prompts to content block arrays
• 5 new test cases for prepend mode behavior (12 total tests, all passing)

Dashboard

• Added mode toggle button (Replace/Prepend) in the project system prompt settings
• New POST /:projectId/system-prompt-mode endpoint to toggle mode
• Updated API endpoint to accept and validate system_prompt_mode field
• Dynamic description text updates based on selected mode

Documentation

• Updated ADR-034 with prepend mode schema, behavior, and resolved the "all-or-nothing" negative consequence
• Updated API Reference with new system_prompt_mode field documentation
• Updated Dashboard Guide with mode selector description
• Added changelog entry

Test plan

• TypeScript typecheck passes (bun run typecheck)
• All 12 system prompt override tests pass (7 replace + 5 prepend)
• Full test suite: no new failures introduced (18 pre-existing failures in Docker/CLI integration tests)
• Manual: verify mode toggle in dashboard UI
• Manual: verify prepend mode correctly combines prompts in proxy
• Run migration 023 on staging database

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