feat(install): add native Roo Code install target

[Fmarzochi]

Mission

Add Roo Code (the popular open-source AI coding agent for VS Code, formerly Roo Cline) as a native install target in scripts/lib/install-targets/, so egc install --target roocode writes EGC guidelines into Roo Code's discovery path instead of requiring manual setup.

Why this is worth doing

Roo Code reads project-level rules from the .roo/rules/ directory inside the repository, a simple documented folder of instruction files, which makes this one of the simplest possible adapters and opens EGC to one of the largest VS Code agent communities. Please verify the current path against the official Roo Code docs before implementing.

Where to look

• scripts/lib/install-targets/junie-project.js -- the newest minimal project adapter (merged this week in feat(install): add native JetBrains Junie install target #849), closest template
• scripts/lib/install-targets/trae-project.js -- another minimal single-directory project adapter
• scripts/lib/install-targets/helpers.js -- createInstallTargetAdapter
• scripts/lib/install-targets/registry.js -- register the new adapter in the ADAPTERS array
• tests/lib/install-targets.test.js -- mirror the existing adapter test shape
• docs/spec/integration-tiers.md + tests/spec/integration-tiers.test.js -- EGC has a docs contract test: every new target must be added to the tier table AND to the EXPECTED_HARNESSES list, or the full CI matrix fails

Acceptance criteria

• roocode-project.js adapter with the verified .roo/rules/ layout
• Registered in registry.js (ADAPTERS array + imports)
• docs/spec/integration-tiers.md and EXPECTED_HARNESSES updated (see docs contract above)
• Tests added mirroring existing install-target coverage
• node tests/run-all.js passes locally
• PR description cites the Roo Code doc page used to verify the path

Not required

• No hook wiring, no Python changes -- scoped entirely to the JS install-target layer