fix: expand @-imports when syncing personalization from CLAUDE.md

[MattBro]

Problem

PostHog Code's Settings → Personalization "Sync from AGENTS.md / CLAUDE.md" toggle (added in #3286) reads the first non-empty of ~/.agents/AGENTS.md, ~/.codex/AGENTS.md, ~/.claude/AGENTS.md, ~/.claude/CLAUDE.md and mirrors it into personalization.

Many users' ~/.claude/CLAUDE.md is a stub that only composes Claude Code @-imports (e.g. @./claude/engineering.md). Reading the file plainly ships the stub and none of the actual rules the imports pull in, so those users sync an almost-empty personalization.

Two wrinkles make a naive fix wrong:

• ~/.claude/CLAUDE.md is often a symlink into a dotfiles repo. Its relative imports resolve against the symlink target's directory, not ~/.claude.
• Imports can be recursive and can form cycles, and a mention of a path inside a code span/block is not an import.

Changes

getUserAgentInstructions in packages/workspace-server/src/services/os/os.ts now expands @-imports before applying the length cap, matching Claude Code's memory-import behavior (docs: https://code.claude.com/docs/en/memory):

• Resolves the realpath of the instructions file first, and resolves each @path relative to the directory of the file that contains it (after realpath), so symlinked stubs resolve their relative imports against the real dotfiles directory.
• Supports relative, absolute, and ~/ import paths.
• Recursively inlines imported file contents, capped at 4 hops deep, with a visited-set cycle guard keyed on realpaths.
• A missing, unreadable, or cyclic import leaves its @path reference in place rather than throwing.
• Skips imports inside fenced code blocks and single-backtick inline code spans.
• Applies the existing 20k USER_AGENT_INSTRUCTIONS_MAX_LENGTH cap after expansion, preserving the truncated flag semantics.
• Files with no imports are byte-for-byte unchanged.

Divergences from Claude Code's import spec

• Code handling follows CommonMark closely: fenced blocks track marker character and length (a closer must be at least as long, same character, no info string), indented code blocks (4+ spaces or a tab after a blank line) stay literal, and inline code spans match equal-length backtick runs, so multi-backtick spans keep their contents literal. One simplification: an indented line without a preceding blank line (e.g. a list continuation) is treated as prose and its imports expand, since distinguishing lazy continuation from code needs a full block parser.
• Import tokens are recognized as @ preceded by start-of-line or whitespace, then a run of non-whitespace characters. This intentionally avoids treating foo@bar (e.g. emails) as imports; a trailing punctuation character on a bare path is included in the path rather than stripped.

How did you test this?

Added unit tests in packages/workspace-server/src/services/os/os.test.ts covering: no-imports passthrough, single import, nested import, cycle guard, missing import, inline-code-span literal, fenced-code-block literal, max-depth cutoff, symlinked root file with a relative import resolving against the real directory, and the length cap applied after expansion, fence length/info-string rules (shorter fence inside a longer one, tilde vs backtick isolation), indented code blocks (blank-line context, tab indent, list continuations still expanding, three-space boundary), and multi-backtick / unmatched-run inline spans.

• pnpm exec vitest run src/services/os/os.test.ts — 30 passed
• Full @posthog/workspace-server suite — 697 passed
• pnpm --filter @posthog/workspace-server typecheck — clean
• biome check on the changed files — clean

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[trunk-io]

😎 Merged successfully - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit f9386f2.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Merge branch 'main' into matt/personaliz..." | Re-trigger Greptile}

[MattBro]

/trunk merge