feat: add PLANNOTATOR_DATA_DIR env var to customize data directory

[IstPlayer]

Summary

Add a PLANNOTATOR_DATA_DIR environment variable that allows users to relocate the Plannotator data directory (default: ~/.plannotator) to any path they prefer.

Motivation

Plannotator currently hardcodes ~/.plannotator/ as the base data directory in multiple places — storage.ts, config.ts, draft.ts, improvement-hooks.ts, sessions.ts, codex-review.ts, tour-review.ts, browser.ts, and pr-gitlab.ts. This clutters the home directory and conflicts with XDG conventions (XDG_DATA_HOME, XDG_CACHE_HOME, etc.) that many users follow to keep their home directories clean.

The PASTE_DATA_DIR env var already exists as precedent for customizing sub-paths. This PR extends that pattern to the entire data directory.

Changes

• New file: packages/shared/data-dir.ts — shared helper getPlannotatorDataDir() that checks PLANNOTATOR_DATA_DIR first (with ~ expansion), falls back to ~/.plannotator
• 9 source files updated: all hardcoded join(homedir(), ".plannotator", ...) calls replaced with getPlannotatorDataDir()
• Unused homedir imports removed where possible

Usage

# Move all Plannotator data to ~/.cache
export PLANNOTATOR_DATA_DIR=~/.cache/plannotator

# Or to an XDG-compliant location
export PLANNOTATOR_DATA_DIR=$XDG_DATA_HOME/plannotator

Backward Compatibility

Default behavior is unchanged — when PLANNOTATOR_DATA_DIR is absent or empty, the data directory remains ~/.plannotator.

[backnotprop]

Thanks for the PR @IstPlayer — clean approach and the right default behavior (nothing changes unless you explicitly set the env var).

I pushed a fix commit (46df9f7a) addressing the two files that still had hardcoded ~/.plannotator paths:

• apps/opencode-plugin/index.ts — getPlanBackingPath() now uses getPlannotatorDataDir() instead of homedir() + ".plannotator". Without this, OpenCode's active plan backing file would ignore PLANNOTATOR_DATA_DIR.
• apps/vscode-extension/src/extension.ts — The IPC_REGISTRY path now resolves through the same env var logic. The VS Code extension is a standalone build (can't import from @plannotator/shared), so the resolution logic is inlined. Without this fix, the extension and server would disagree on the IPC registry location when PLANNOTATOR_DATA_DIR is set.
• packages/shared/package.json — Added the "./data-dir" export so the OpenCode plugin can import it as @plannotator/shared/data-dir.
• AGENTS.md — Documented PLANNOTATOR_DATA_DIR in the env var table.

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For context: There are two other open PRs tackling the same problem with different approaches:

• feat(paths): use XDG base directories instead of ~/.plannotator #568 (@cwrau) — Full XDG Base Directory support with getDataBase()/getStateBase()/getConfigBase() and an existsSync(~/.plannotator) legacy guard. Gets the XDG categories right (state vs cache) but adds complexity.
• Adds XDG Base Directory support (backward compatible) #612 (@Joao-O-Santos) — Kitchen-sink approach with PLANNOTATOR_*_DIR + XDG + legacy fallback. Most comprehensive but has some issues with inline path resolution in paste service/VS Code defaulting to wrong paths.

This PR (#795) is the simplest and safest of the three — one env var, opt-in only, zero risk for anyone who doesn't set it. The co-author trailers on the fix commit credit @cwrau and @Joao-O-Santos since their work helped identify the full scope of files that needed updating.