fix(generate): stop creating empty shared config files

[dyoshikawa]

Background

rulesync generate left untracked, empty files behind:

?? .antigravity/settings.json   # {}
?? .devin/config.json           # { "mcpServers": {} }
?? .factory/settings.json       # {}

These are shared config files rulesync merges into rather than owns, so they are deliberately excluded from rulesync gitignore via DERIVED_PATHS_NOT_GITIGNORED. The result was that every generate run handed the user a file to manage without putting anything in it.

This PR takes Option B from the issue (do not emit an empty merge-target file), which preserves the documented ownership model and needs no .gitignore change.

Changes

Two causes, both fixed:

1. readOrInitializeFileContent created the file as a side effect

readOrInitializeFileContent(filePath, initial) wrote initial to disk when the file was missing, so a merge-target file was materialized before writeAiFiles ever decided whether it had anything to write.

Every generator now reads with (await readFileContentOrNull(filePath)) ?? <default> instead — the pattern the copilot / zed / qwencode permissions generators had already adopted for exactly this reason. Three call sites that discarded the result entirely (antigravity/devin/grokcli hooks) are simply dropped. The util itself is removed so the eager-write path cannot come back.

Side benefit: --dry-run / --check no longer touch the filesystem in ~30 generators.

2. writeAiFiles created files for empty payloads

FeatureProcessor#writeAiFiles now skips a file that does not exist yet when:

• the generated payload is empty, and
• the path is one of the shared, user-managed config files — AiFile#shouldSkipCreationWhenPayloadEmpty().

An already-existing file is still rewritten, so user-authored content is never dropped, and every other file is still created regardless.

The path list moves to src/constants/shared-config-paths.ts as SHARED_USER_MANAGED_CONFIG_PATHS, and DERIVED_PATHS_NOT_GITIGNORED is now derived from it — one source of truth for "rulesync merges into this file but does not own it", so the gitignore exclusions and the creation rule cannot drift apart. (The predicate deliberately does not key off isDeletable(): that flag also protects global-scope files and files the orphan sweep cannot rediscover, which are unrelated concerns.)

fileContentIsEmptyPayload in src/utils/content-equivalence.ts decides emptiness on the parsed document (JSON/JSONC/YAML/TOML), so {}, {"permissions":{}} and {"mcpServers":{}} all count as empty while any scalar counts as content. It is deliberately conservative — a non-empty array, an object with a non-plain prototype (Date/TomlDate, or a __proto__ entry that jsonc-parser resolves by replacing the prototype), a value already on the current path, and anything unparseable all count as content, because "not empty" only ever means "write the file".

Verification

Generating every target/feature from this repo's .rulesync/ into a clean directory, before vs. after — exactly the three paths from the issue, and nothing else:

- ./.antigravity/settings.json   # {}
- ./.factory/settings.json       # {}
- ./.vscode/settings.json        # {}

.devin/config.json is still written in that particular run because it genuinely carries mcpServers content; with an empty MCP payload it is skipped too.

Tests added:

• src/e2e/e2e-permissions.spec.ts — the regression test the issue asked for: an empty permissions payload leaves .antigravity/settings.json, .factory/settings.json and .vscode/settings.json absent.
• src/constants/shared-config-paths.test.ts — path matching, including any-depth matches and partial-segment non-matches.
• src/utils/content-equivalence.test.ts — JSON/JSONC/YAML/TOML, comment-only YAML, null, arrays, __proto__, a self-referential YAML anchor, and a date-only TOML document.
• src/types/feature-processor.test.ts — the four writeAiFiles branches, delegating to the real AiFile.prototype predicate rather than a stub.

pnpm cicheck passes.

Closes #2430

[dyoshikawa]

@dyoshikawa Thank you!