feat: Group prompts under roles

[thykel]

What this changes

Every LLM call now names a role: planner, advisor, implementer, pr_author, and 7 more. A role is two files side by side in lib/opilot/prompts/:

• <role>.yml holds the tool grant (read/write), mcp, model (heavy/light), memory (session/none) and the role's charter.
• <role>.rb is the role's prompt module (Prompts::Planner.plan, Prompts::PrAuthor.fix_ci, …).

Text and helpers that several roles use are in prompts/_shared.rb and prompts/_blocks/*.md.

Why

Before this PR, each call site chose its tool grant, its model and its session by hand. Each prompt also stated its grant by hand: READ_ONLY appeared in 10 prompts, and the write rules in 3. Nothing checked that the prompt text, the grant and the session agreed.

How it works

• Helpers#llm(:role, prompt, …) is the only way to call the model. The role decides the tools and the model.

• A memory: none role refuses a session file. The health check's independence from chat is now enforced by the code, not by a comment.

• A builder returns a Prompts::Prompt that is tagged with its role. llm refuses a prompt sent under another role.

• Every prompt that orients the model opens with Prompts.charter: the role's charter, then the rules of its grant. A read role gets READ_ONLY, and a write role gets WRITE_GRANT. These rules are derived from the grant, so a prompt cannot state a grant that its role does not have.

• The runner sends X-Harness-Role. server.js loads the same YAML files and refuses these requests:

  • a request with no role, or with an unknown role;
  • a request with no tool grant, because pi's default tools include write;
  • a grant that the role does not allow.

  This check catches a bug in the runner. It does not stop a compromised runner, because the runner chooses both the role and the grant.

Prompt text changes

Only commit e1d7ff6 changes prompt text:

• The "You are the WRITER" and "You are opilot, an AI code assistant…" openings are replaced by the role's charter and one context sentence.
• The grant rules move to the top of each prompt. The no-commit rule for write roles had three wordings. It now has one.
• commit_subject gains READ_ONLY. propose and propose_feedback gain the write rules, including the git rm/git clean note.

All other commits are moves. A snapshot of all 19 builders over 35 argument combinations renders byte-identical across each of them.

Before merge

• Run a real ./opilot dev plan <id> on a known work package, and compare the plan with one made before this change. No live model run has checked the new prompt text.
• The harness image must be rebuilt, because it now copies lib/opilot/prompts/*.yml. ./opilot rebuilds on every start.

Tests

• roles_test.rb pins every role's tuple and checks every grant against ALLOWED_TOOL_GRANTS. It also fails when a call site bypasses llm.
• prompts_test.rb renders every builder. It checks the charter, checks that the builder's own grant block appears exactly once and the other block never appears, and checks the file pairs.
• test/js/roles_test.js checks the server's parser and its role check against the real role files.

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