fix(engram): prevent double MCP registration in Antigravity and support Linuxbrew path

[eezzekl]

🔗 Linked Issue

Closes #797

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🏷️ PR Type

What kind of change does this PR introduce?

• type:bug — Bug fix (non-breaking change that fixes an issue)
• type:feature — New feature (non-breaking change that adds functionality)
• type:docs — Documentation only
• type:refactor — Code refactoring (no functional changes)
• type:chore — Build, CI, or tooling changes
• type:breaking-change — Breaking change (fix or feature that changes existing behavior)

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📝 Summary

• Stop writing a duplicate mcp_config.json in the Antigravity plugin directory; Engram MCP is now registered exactly once, in the global CLI config.
• Recognize the Linuxbrew Engram binary path (/home/linuxbrew/.linuxbrew/bin/engram) as a stable path so auto-injection works on Linux installs via Linuxbrew.
• Always pass --tools=agent when launching the Engram MCP server for Antigravity, instead of stripping it for that agent.

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📂 Changes

File / Area	What Changed
internal/components/engram/inject.go	Removed the Antigravity-only branch that dropped --tools=agent; removed duplicate plugin-scope mcp_config.json write; added Linuxbrew path to isStableHomebrewEngramPath
internal/components/engram/inject_test.go	Updated assertions to expect --tools=agent and to expect no plugin-scope mcp_config.json file

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🧪 Test Plan

• Unit tests pass (go test ./...)
• Go format passes (go run ./internal/gofmtcheck)
• E2E tests pass (cd e2e && ./docker-test.sh)
• Manually tested locally

CI will run the full suite; local go toolchain wasn't available in this environment to run it beforehand.

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✅ Contributor Checklist

• PR is linked to an issue with status:approved
• PR stays within 400 changed lines, or I have requested/obtained maintainer-applied size:exception with rationale documented
• I have added the appropriate type:* label to this PR
• Unit tests pass (go test ./...)
• Go format passes (go run ./internal/gofmtcheck)
• E2E tests pass (cd e2e && ./docker-test.sh)
• I have updated documentation if necessary
• My commits follow Conventional Commits format
• My commits do not include Co-Authored-By trailers

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💬 Notes for Reviewers

Local go was not runnable in this environment (permission denied on the go binary), so unit/format/E2E checks weren't executed locally before opening this PR — relying on CI to confirm. Branch was rebased onto latest origin/main before opening (no conflicts in internal/components/engram/inject.go despite other commits touching that file in between).

Summary by CodeRabbit

• Bug Fixes
  • Improved Antigravity integration with Engram by aligning MCP configuration behavior with other supported agents.
  • Antigravity now includes the required agent tools configuration for Engram commands.
  • Prevented unnecessary plugin-level MCP configuration files from being created.
  • Added support for recognizing Engram installations managed through Linuxbrew.

[coderabbitai]

📝 Walkthrough

Walkthrough

Antigravity Engram registration now uses the standard MCP arguments, avoids writing a plugin-level mcp_config.json, and recognizes the Linuxbrew Engram binary path. Tests validate the updated CLI configuration and absence of the plugin MCP file.

Changes

Engram registration

Layer / File(s)	Summary
Antigravity MCP registration internal/components/engram/inject.go, internal/components/engram/inject_test.go	Antigravity uses the standard overlay arguments, no longer writes a plugin-level MCP configuration, and tests require --tools=agent in the CLI configuration while asserting the plugin MCP file is absent.
Linuxbrew path detection internal/components/engram/inject.go	The stable Engram path allowlist includes /home/linuxbrew/.linuxbrew/bin/engram.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested labels: type:bug

Suggested reviewers: alan-thegentleman

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes match #797 by removing plugin-level mcp_config.json, preserving --tools=agent, and recognizing the Linuxbrew Engram path.
Out of Scope Changes check	✅ Passed	No clearly unrelated code changes are shown; the edits stay focused on Antigravity Engram injection and its tests.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main changes: removing duplicate MCP registration in Antigravity and adding Linuxbrew path support.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[Alan-TheGentleman]

Closing as duplicate of #1643. That carrier includes the Linuxbrew and tool-profile behavior from this PR plus the transactional ownership migration, idempotency, and recovery tests needed for #797. Please continue review and the remaining rollback fix there.