workflows: add system_prompt to reviewer + integrator frontmatter; drop obsolete coder/overseer

Goal

Ensure every live workflow in intent/Omni/Ide/Workflows/ has a system_prompt line in its YAML frontmatter so agents get the role/protocol guidance via the real pipeline instead of being baked into the user prompt. Drop the obsolete unused ones.

Current state (confirmed)

Live workflows (referenced by intent/Omni/Ide/dev-review-release.sh + intent/Omni/Ava/Telegram/Developer.hs):

• dev.md — has system_prompt: dev-system.md (OK)
• reviewer.md — has frontmatter but no system_prompt
• integrator.md — has frontmatter but no system_prompt

Obsolete (no code references; grep confirms only overseer.md references coder.md inside its own body):

• coder.md — no frontmatter at all, only a fenced yaml block inside ## Spawn Configuration which nothing parses
• overseer.md — no frontmatter, only prose

The resolveSystemPrompt helper (intent/Omni/Agent.hs:570) already supports either an inline string or a file path (.md/.txt/contains /). So the new frontmatter entries can point at sibling *-system.md files and be loaded automatically.

Acceptance criteria

1. Create intent/Omni/Ide/Workflows/reviewer-system.md with a concise role+protocol prompt:

• role (review changes for correctness, style, tests, build hygiene)
• read-only expectations; no commits
• output format (what to write back to the task)
• how to use task system (task comment, task update review)

2. Create intent/Omni/Ide/Workflows/integrator-system.md with a concise role+protocol prompt:

• role (integrate reviewed patches, resolve trivial conflicts, rebase if needed)
• commit/merge protocol (one task = one commit; amend + force-push for revisions per the Gerrit-like workflow, t-565 epic)
• how to report completion

3. Edit intent/Omni/Ide/Workflows/reviewer.md frontmatter: add system_prompt: reviewer-system.md next to the existing keys.

4. Edit intent/Omni/Ide/Workflows/integrator.md frontmatter: add system_prompt: integrator-system.md.

5. Delete intent/Omni/Ide/Workflows/coder.md and intent/Omni/Ide/Workflows/overseer.md. They are unreferenced by any code path.

6. Verify with: rg -l 'Workflows/(coder|overseer)' intent should return nothing after the deletion.

Out of scope

• No changes to dev.md / dev-system.md (already correct).
• No new spawn logic; existing dispatch path already passes file path to agentd run which triggers parseWorkflowFrontmatter + resolveSystemPrompt.
• Don't restructure Developer.hs or the release script.

Testing

• agentd run Omni/Ide/Workflows/reviewer.md 'smoke test' should succeed and the agent's context should include the text from reviewer-system.md.
• Same for integrator.md.
• Build should still pass.