feat: add repo agent instructions

[jeremymcs]

Summary

• Add per-repository agent instructions to watched repo settings and persist them in SQLite/in-memory storage.
• Inject those instructions into issue-work and PR babysitter prompts, including docs assessment, merge-conflict repair, and code-owner fallback prompts.
• Add a settings textarea for editing instructions per repo.
• Update the Settings screenshot to show the repo-specific agent instructions field.

Verification

• npm run test -- server/backgroundJobHandlers.test.ts server/issueWorkAgent.test.ts server/routes.test.ts server/storage.test.ts server/memoryStorage.test.ts server/babysitter.test.ts
• npm run check
• BASE_URL=http://127.0.0.1:5011 node script/screenshot.mjs
• Visual check: Settings screenshot shows repo Agent instructions without layout overlap.

Related Issue

• None

[github-actions]

📄 Documentation Review Reminder

This PR changes source files but does not update any public documentation.

Changed source files:

client/src/pages/settings.tsx
server/babysitter.test.ts
server/babysitter.ts
server/backgroundJobHandlers.test.ts
server/backgroundJobHandlers.ts
server/issueWorkAgent.test.ts
server/issueWorkAgent.ts
server/memoryStorage.test.ts
server/memoryStorage.ts
server/routes.test.ts
server/routes.ts
server/sqliteStorage.ts
server/storage.test.ts
shared/models.ts
shared/schema.ts

Please check if documentation in docs/public/ needs to be updated:

• No documentation update needed
• Documentation updated in this PR
• Follow-up documentation issue created

Public docs are Markdown files in docs/public/. They are automatically
rendered to HTML and deployed to GitHub Pages when merged to main.

Available docs: getting-started.md, pr-babysitter.md, agent-dispatch.md, configuration.md, pr-questions.md