refactor(help): update engram --help output to show the right configuration example for engram mcp addition

[pablon]

The confusing part is engram --help text output, not the setup flow itself.

engram setup opencode already writes the correct OpenCode configuration format. The proposed fix aligns engram --help text output with the actually supported OpenCode config.

🔗 Linked Issue

Closes #88

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🏷️ PR Type

• type:bug — Bug fix
• type:feature — New feature
• type:docs — Documentation only
• type:refactor — Code refactoring (no behavior change)
• type:chore — Maintenance, dependencies, tooling
• type:breaking-change — Breaking change

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📝 Summary

engram --help currently shows wrong directions to the user.

This PR makes engram --help show the correct format for the user to configure mcp.engram in opencode.json without errors.

📂 Changes

File	Change
cmd/engram/main.go	Shows correct directions to configure mcp.engram in engram --help output

🧪 Test Plan

• Unit tests pass locally: go test ./...
• E2E tests pass locally: go test -tags e2e ./internal/server/...
• Manually tested the affected functionality

1. Without the suggested change opencode mcp list shows:

┌  MCP Servers
Error: Configuration is invalid at ~/.config/opencode/opencode.json
↳ Invalid input mcp.engram

opencode does not open and also shows the above error ❌

2. With the suggested change opencode mcp list shows:

┌  MCP Servers
│
●  ✓ engram connected
│      engram mcp --tools=agent
...

opencode opens without errors ✅

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🤖 Automated Checks

These run automatically and all must pass before merge:

Check	What it verifies	Status
Check Issue Reference	PR body contains Closes #N / Fixes #N / Resolves #N	⏳
Check Issue Has status:approved	Linked issue has status:approved label	⏳
Check PR Has type:* Label	PR has exactly one type:* label	⏳
Unit Tests	go test ./... passes	⏳
E2E Tests	go test -tags e2e ./internal/server/... passes	⏳

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✅ Contributor Checklist

• I linked an approved issue above (Closes #N)
• I added exactly one type:* label to this PR
• I ran unit tests locally: go test ./...
• I ran e2e tests locally: go test -tags e2e ./internal/server/...
• Docs updated (if behavior changed)
• Commits follow conventional commits format
• No Co-Authored-By trailers in commits

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💬 Notes for Reviewers

[Alan-TheGentleman]

Thanks for jumping on a real problem. I am requesting changes because the current fix is not solid enough to merge as-is: the proposed example mixes format assumptions, introduces an extra field that should not be documented casually, and does not fully resolve the OpenCode-facing confusion. Rework this from the actual supported config examples and then we can review it again.

[pablon]

Thanks for jumping on a real problem. I am requesting changes because the current fix is not solid enough to merge as-is: the proposed example mixes format assumptions, introduces an extra field that should not be documented casually, and does not fully resolve the OpenCode-facing confusion. Rework this from the actual supported config examples and then we can review it again.

Reviewed and re-edited 👍🏻