fix(agent): include posthog-code/ branch prefix in global instructions

[adboio]

Summary

• The posthog-code/ branch prefix convention was only injected into the auto-create-PR cloud prompt (agent-server.ts:1621).
• When a cloud session ran through the review-only !shouldAutoCreatePr branch and the user later asked the agent to create a branch, the agent had no convention loaded and fell back to its own default (<user>/<slug>).
• Move the prefix instruction into the globally-appended BRANCH_NAMING block in packages/agent/src/adapters/claude/session/instructions.ts so it applies to every prompt branch — cloud auto-PR, cloud review-only, no-repo, and local.

Test plan

• Start a cloud task in review-only mode, then ask the agent to create a branch — verify it uses the posthog-code/ prefix
• Start a cloud task in auto-PR mode — verify the prefix still appears
• Start a local task and ask for a branch — verify the prefix still appears

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 1
packages/agent/src/adapters/claude/session/instructions.ts:6
**Instruction duplicated, not moved**

The PR description says this is a *move*, but the identical `posthog-code/` prefix instruction still exists in `agent-server.ts:1621` (the auto-PR branch). Now both prompts carry it, violating OnceAndOnlyOnce. The line in `agent-server.ts` that reads `1. Create a new branch prefixed with \`posthog-code/\` (e.g. \`posthog-code/fix-login-redirect\`) based on the work done` should be simplified to remove the redundant prefix hint and just say e.g. `1. Create a new branch based on the work done` (relying on the global instruction for the naming convention).

_{Reviews (1): Last reviewed commit: "fix(agent): include posthog-code/ branch..." | Re-trigger Greptile}

[greptile-apps]

Instruction duplicated, not moved

The PR description says this is a move, but the identical posthog-code/ prefix instruction still exists in agent-server.ts:1621 (the auto-PR branch). Now both prompts carry it, violating OnceAndOnlyOnce. The line in agent-server.ts that reads 1. Create a new branch prefixed with \posthog-code/` (e.g. `posthog-code/fix-login-redirect`) based on the work doneshould be simplified to remove the redundant prefix hint and just say e.g.1. Create a new branch based on the work done` (relying on the global instruction for the naming convention).

Prompt To Fix With AI

This is a comment left during a code review.
Path: packages/agent/src/adapters/claude/session/instructions.ts
Line: 6

Comment:
**Instruction duplicated, not moved**

The PR description says this is a *move*, but the identical `posthog-code/` prefix instruction still exists in `agent-server.ts:1621` (the auto-PR branch). Now both prompts carry it, violating OnceAndOnlyOnce. The line in `agent-server.ts` that reads `1. Create a new branch prefixed with \`posthog-code/\` (e.g. \`posthog-code/fix-login-redirect\`) based on the work done` should be simplified to remove the redundant prefix hint and just say e.g. `1. Create a new branch based on the work done` (relying on the global instruction for the naming convention).

How can I resolve this? If you propose a fix, please make it concise.