feat: add --update mode, --meta/--meta-plugin flags, streaming diff

- Add --update mode to modify tags and metadata for existing items by ID
- Add --meta key=value flag to set metadata during save/update
- Add --meta key (bare) to delete metadata keys or filter by existence
- Add --meta-plugin/-M name:{json} flag for plugin options via CLI
- Env meta plugin now uses options from --meta-plugin instead of only env vars
- Stream decompressed content to diff via /dev/fd pipes (no temp files)
- Wire --list-format CLI arg to settings (was parsed but ignored)
- Allow --info to accept tags (was restricted to numeric IDs only)
- Change DB meta filtering to HashMap<String, Option<String>> for exact match + key existence
- Fix fcntl error checking in diff pre_exec
- Fix README inaccuracies (delete by tag, nonexistent --digest flag, meta plugin key names)

README.md

@@ -149,8 +149,8 @@ keep --list
 # Get item details
 keep --info greeting
 
-# Delete by tag
-keep --delete greeting
+# Delete by ID
+keep --delete 1
 ```
 
 ### Real-World Examples
@@ -192,8 +192,8 @@ echo "data" | keep my-tag
 # Save with multiple tags and metadata
 cat report.pdf | keep --save report --meta project=alpha --meta env=prod
 
-# Specify compression and digest algorithm
-echo "data" | keep --save my-tag --compression gzip --digest sha256
+# Specify compression
+echo "data" | keep --save my-tag --compression gzip
 ```
 
 Tags and metadata make items easy to find later. Tags are simple identifiers; metadata is key-value pairs.
@@ -364,7 +364,7 @@ Metadata is automatically extracted when saving items.
 | `env` | `*` | Capture `KEEP_META_*` environment variables |
 | `magic_file` | `file_type` | File type detection (requires `magic` feature) |
 | `text` | `text_line_count`, `text_word_count` | Line and word counts |
-| `user` | `uid`, `user`, `gid`, `group` | Current user info |
+| `user` | `user_uid`, `user_name`, `user_gid`, `user_group` | Current user info |
 | `shell` | `shell` | Current shell path |
 | `shell_pid` | `shell_pid` | Shell process ID |
 | `keep_pid` | `keep_pid` | Keep process ID |
@@ -376,8 +376,11 @@ Metadata is automatically extracted when saving items.
 | `cwd` | `cwd` | Current working directory |
 
 ```sh
-# Use specific plugins
-echo "data" | keep --save tag --meta-plugins "digest,text,user"
+# Use specific plugins (repeatable)
+echo "data" | keep --save tag --meta-plugin digest --meta-plugin text --meta-plugin user
+
+# Pass options to a plugin via JSON
+echo "data" | keep --save tag --meta-plugin 'tokens:{"options":{"min_length":"2"}}'
 
 # Capture custom metadata via environment
 KEEP_META_project=alpha echo "data" | keep --save tag
@@ -395,7 +398,7 @@ KEEP_META_build=1234 echo "data" | keep --save tag --meta env=staging
 | `KEEP_DIR` | Storage directory | `~/.keep` |
 | `KEEP_CONFIG` | Config file path | `~/.config/keep/config.yml` |
 | `KEEP_COMPRESSION` | Compression algorithm | `lz4` |
-| `KEEP_META_PLUGINS` | Meta plugins to use | `env` |
+| `KEEP_META_PLUGINS` | Meta plugins to use (JSON format: `name[:{json}]`, comma-separated) | `env` |
 | `KEEP_FILTERS` | Default filter chain | none |
 | `KEEP_LIST_FORMAT` | List column format | built-in defaults |
 | `KEEP_SERVER_ADDRESS` | Server bind address | `127.0.0.1` |
@@ -758,16 +761,16 @@ This means client behavior is consistent with local mode — the same compressio
 Client save uses a 3-thread streaming pipeline for constant memory usage regardless of data size:
 
 ```
-┌──────────────┐     OS pipe      ┌────────────────┐
+┌───────────────┐     OS pipe      ┌────────────────┐
 │ Reader thread ├──────────────────┤ Streamer thread│
-│              │  (compressed     │                │
-│ stdin → tee  │   bytes)         │ pipe → POST    │
-│    → hash    │                  │   (chunked)    │
-│    → compress│                  │                │
-└──────────────┘                  └────────────────┘
-        │                                │
-        ▼                                ▼
-    stdout +                    Server stores blob
+│               │  (compressed     │                │
+│ stdin → tee   │   bytes)         │ pipe → POST    │
+│    → hash     │                  │   (chunked)    │
+│    → compress │                  │                │
+└───────────────┘                  └────────────────┘
+        │                                  │
+        ▼                                  ▼
+    stdout +                       Server stores blob
     SHA-256 digest
 ```