feat: add fragments support for custom agent instructions

[vtemian]

Summary

• Port the fragments pattern from micode, allowing users to inject custom instructions into agent prompts via config files
• Two-layer configuration: global (~/.config/opencode/octto.json) + project (.octto/fragments.json)
• Fragments are merged (global first, project appends) and formatted as XML blocks prepended to agent system prompts
• Includes typo detection with Levenshtein distance suggestions for unknown agent names

Test plan

• Run bun test - all 150 tests pass
• Create test config with visible fragments (emojis)
• Verify fragments appear in octto agent output
• Verify fragments appear in probe agent questions
• Test project-level fragments merge with global

Closes #3

---

Summary by cubic

Adds “fragments” support to inject custom instructions into agent system prompts via config, merged globally and per-project and rendered as an XML block. Fulfills Linear #3 by enabling per-agent prompt customization with typo detection for agent names.

• New Features

  • Add fragments in global ~/.config/opencode/octto.json and project .octto/fragments.json.
  • Merge order: global first, project appends.
  • Render fragments as a block with bullet points, prepended to the agent prompt.
  • Warn on unknown agent names and suggest likely matches via Levenshtein distance.

• Bug Fixes

  • Inject fragments at plugin load (agent source), ensuring brainstorming sessions also use the merged fragments.
  • Remove duplicate injection in the config hook to prevent repeated instructions.

^{Written for commit 9758f1b. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 9 files

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="src/index.ts">

<violation number="1" location="src/index.ts:21">
P2: Custom-configured agent prompts get fragment prefixes twice (once when mutating customConfig, again in config()), leading to duplicated instructions. Apply fragment injection in only one place or avoid mutating customConfig.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}