docs: clarify dirs and since flag

AGENTS.md

@@ -37,12 +37,13 @@ This document equips future agents with the current mental model for the `studip
    - Normalizes path components and uses `NameRegistry` to avoid collisions, guaranteeing human-readable yet unique names.
    - Checks file state (size, modified timestamp, checksum) against `state.toml` to skip unchanged files; downloads stream to `*.part` before rename.
    - Records remote metadata + local path hints in state. `--dry-run` reports actions without touching disk; `--prune` (plus non–dry-run) deletes stray files/dirs with `walkdir`.
+   - `--since <ISO8601>` is accepted for future incremental sync work; at the moment it is recorded in logs/state but no API filters are issued.
 5. HTTP errors propagate via `anyhow`, but 401/403 currently surface as generic failures—production UX should point users to `studip-sync auth`.
 
 ## Configuration & State
 
-- Config path: `${XDG_CONFIG_HOME:-~/.config}/studip-sync/config.toml`. Example keys: `base_url`, `jsonapi_path`, `basic_auth_b64`, `download_root`, `max_concurrent_downloads`.
-- State path: `${XDG_DATA_HOME:-~/.local/share}/studip-sync/state.toml`.  
+- Config path: `${XDG_CONFIG_HOME:-~/.config}/studip-sync/config.toml`. Override with `--config-dir` when needed. Example keys: `base_url`, `jsonapi_path`, `basic_auth_b64`, `download_root`, `max_concurrent_downloads`.
+- State path: `${XDG_DATA_HOME:-~/.local/share}/studip-sync/state.toml`. `--data-dir` relocates this tree (and the default `downloads/` folder). Explicitly setting `download_root` decouples downloads from the data dir; otherwise it defaults to `<data-dir>/downloads`.  
   - `profiles.<name>.user_id` caches `/users/me`.  
   - `profiles.<name>.semesters.<key>` stores semester IDs/titles/keys.  
   - `profiles.<name>.courses.<id>` keeps display names + `last_sync`.  

README.md

@@ -13,9 +13,9 @@
 
 ## Directory Layout & Data Files
 
-- Config lives under `${XDG_CONFIG_HOME:-~/.config}/studip-sync/config.toml`. A `default` profile is created automatically and stores the `basic_auth_b64`, base URL, JSON:API path, download root, etc.
-- State is cached in `${XDG_DATA_HOME:-~/.local/share}/studip-sync/state.toml` with per-profile sections for user/semester/course/file metadata.
-- Downloads default to `${XDG_DATA_HOME:-~/.local/share}/studip-sync/downloads`, but you can override `download_root` in the config to point anywhere else. Each path segment is sanitized to keep names human-readable yet filesystem-safe.
+- Config lives under `${XDG_CONFIG_HOME:-~/.config}/studip-sync/config.toml`. Override this with `--config-dir` if you want the config somewhere else.
+- State is cached in `${XDG_DATA_HOME:-~/.local/share}/studip-sync/state.toml`; `--data-dir` only changes this location (and anything else the tool stores under data, such as the default downloads folder). Use this when you want the state cache on a different disk but keep the config where it is.
+- `download_root` determines where files land. If omitted, it falls back to `<data-dir>/downloads`, so moving the data dir automatically relocates the default downloads. Setting `download_root` explicitly decouples it from the data dir. Each path segment is sanitized to keep names human-readable yet filesystem-safe.
 
 ## Getting Started
 
@@ -70,9 +70,9 @@ max_concurrent_downloads = 3 # placeholder for future concurrency control
 | `init-config` | Write a default config template (fails if config exists unless forced). | `--force`, `--download-root` |
 | `auth` | Collect username/password, encode them, and save them to the active profile. | `--non-interactive`, `--username`, `--password` |
 | `list-courses` | List cached or freshly fetched courses with semester keys and IDs. | `--refresh` |
-| `sync` | Download files for every enrolled course into the local tree. | `--dry-run`, `--prune`, `--since` *(reserved for future API filters)* |
+| `sync` | Download files for every enrolled course into the local tree. | `--dry-run`, `--prune`, `--since` *(currently just records the user-provided timestamp; the API filtering hook is planned but not implemented yet)* |
 
-Global flags: `--quiet`, `--debug`, `--json`, `-v/--verbose` (stackable), `--config-dir`, `--data-dir`, `--profile`.
+Global flags: `--quiet`, `--debug`, `--json`, `-v/--verbose` (stackable), `--config-dir`, `--data-dir` (state + default downloads), `--profile`.
 
 ## Sync Behavior
 
@@ -83,7 +83,7 @@ Global flags: `--quiet`, `--debug`, `--json`, `-v/--verbose` (stackable), `--con
    - List file refs via `/folders/{id}/file-refs`, normalize filenames, and ensure unique siblings through a `NameRegistry`.
    - Skip downloads when the local file exists and matches the stored checksum / size / remote `chdate`.
    - Stream downloads to `*.part`, hash contents on the fly, then rename atomically to the final path.
-4. Maintain a set of remote files so `--prune` can remove local files that no longer exist remotely (and optionally delete now-empty directories).
+4. Maintain a set of remote files so `--prune` can remove local files that no longer exist remotely (and optionally delete now-empty directories). `--since` is accepted for future incremental sync work and currently acts as an annotation only—no API filters are applied yet.
 5. `--dry-run` prints planned work but never writes to disk.
 
 ## Development Notes