refactor: extract gitignore helpers out of scaffold.ts #882

Context

packages/codev/src/lib/scaffold.ts was originally extracted in Maintenance Run 0004 to deduplicate logic shared between codev init and codev adopt — its header still says "Scaffold utilities for codev init and adopt commands". Since then it has drifted:

• updateGitignore() was added for the adopt path — merging a Codev block into an existing .gitignore. Already not strictly "scaffolding."
• backfillGitignore() was added in Fix #880: gitignore .architect-role.md and add update-time backfill #881 for the update path — repairing stale state in long-lived projects. Definitely not scaffolding.

"Scaffold" implies creating new project structure, but the file now also handles in-place repair of existing projects. The name no longer matches the contents.

Proposed change

Extract gitignore-related helpers into a new file:

packages/codev/src/lib/gitignore.ts

Moves:

• CODEV_GITIGNORE_ENTRIES constant
• FULL_GITIGNORE_CONTENT constant
• createGitignore() (used by init)
• updateGitignore() (used by adopt)
• backfillGitignore() (used by update)
• The internal parseEntryLines() helper
• The BackfillGitignoreResult / BackfillGitignoreOptions / UpdateGitignoreResult types

scaffold.ts keeps only the genuine scaffolding helpers — createUserDirs, copyConsultTypes, copyResourceTemplates, copyRootFiles, copyRoles, copyProtocols, copySkills, createProjectsDir, copyDirRecursive.

Acceptance criteria

• New file packages/codev/src/lib/gitignore.ts contains all gitignore-related exports listed above
• scaffold.ts no longer references gitignore behavior
• Import sites updated (init.ts, adopt.ts, update.ts, the test files)
• Existing test coverage moved to a new gitignore.test.ts (or split out of scaffold.test.ts) — no test deleted or weakened, only relocated
• tsc clean, full test suite green

Out of scope

• No behavior change. This is a pure file move + import update.
• No renaming of the functions themselves.
• Not pruning scaffold.ts further (e.g. moving copyRoles to a roles.ts file). One refactor at a time.

Discovered while

Reviewing PR #881 (issue #880). The file's name vs. its current contents came up during post-merge discussion.