feat: MCP Prompts — workflow templates for common tasks

[OneStepAt4time]

M1.3 — MCP Prompts

Parent: #440 (Roadmap)
Milestone: M1 — MCP Server Complete
Depends on: #441 (tool completeness)

Context

MCP Prompts are pre-built templates that agents can use to execute common workflows. Instead of knowing how to combine 5+ tool calls, the agent uses a prompt template and Aegis fills in the steps.

Prompts to implement

Prompt	Arguments	What it does
implement_issue	issueNumber, workDir, repoOwner, repoName	Creates session, reads issue from GitHub, sends structured implementation prompt
review_pr	prNumber, workDir, repoOwner, repoName	Creates session, reads PR diff, sends review prompt
debug_session	sessionId	Reads transcript + status, generates diagnostic summary

Acceptance criteria

• All 3 prompts implemented via server.prompt()
• prompts/list returns correct metadata
• prompts/get returns properly formatted messages with arguments substituted
• Prompts work end-to-end when called from Claude Code