feat(agent): Instruct agent to follow repo rules and match comment style

[posthog]

Problem

An inbox-beta user (ticket #64146) asked whether they can give the coding agent extra guidance, and reported that generated PRs don't follow their coding style, add lots of unnecessary comments that just explain the fix, and ignore the repo's cursor/claude rules. Giving direction at Create-PR time already shipped (the feedback steering prompt), but nothing made the agent honor the repo's own conventions or match its comment style.

Changes

Added a Repository Conventions block to the agent's appended system-prompt instructions (buildAppendedInstructions → buildSystemPrompt), which is applied to every session (desktop and cloud, including inbox-generated PRs). It tells the agent to:

• Discover and follow repo-level convention files before writing code: CLAUDE.md, AGENTS.md, .cursorrules, and rule files under .cursor/rules/ (*.mdc). Claude Code loads CLAUDE.md/AGENTS.md natively but never reads Cursor rules, so the instruction explicitly points the agent at them.
• Mirror the existing code's style, especially comment density – don't add comments that narrate the change or restate what the code plainly does.

Why: directly addresses the beta feedback that PRs ignore coding style, over-comment, and skip the repo's cursor/claude rules.

How did you test this?

• Added and ran packages/agent/src/adapters/claude/session/instructions.test.ts – asserts the new "Repository Conventions" block is present (referencing .cursor/rules/ and comment density). All 4 tests pass via pnpm exec vitest run.
• Change is confined to a leaf constant file (instructions.ts) with no new imports.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code from an inbox report

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 1ecb403.}