docs: document defaultProject configuration option

Updates README.md to explain the new defaultProject behavior:
- Config example now shows the defaultProject field
- New row in the configuration reference table describing the field,
  its type (optional string), default (none), and behavior (fallback
  when -p omitted, must match a configured path, CLI always overrides)
- Project Resolution section updated to explain the cascading logic:
  CLI flag > config default > all projects
- Init section notes the interactive prompt for multi-project setups
  and the --default-project flag for non-interactive/robot mode

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -91,6 +91,7 @@ Configuration is stored in `~/.config/lore/config.json` (or `$XDG_CONFIG_HOME/lo
     { "path": "group/project" },
     { "path": "other-group/other-project" }
   ],
+  "defaultProject": "group/project",
   "sync": {
     "backfillDays": 14,
     "staleLockMinutes": 10,
@@ -119,6 +120,7 @@ Configuration is stored in `~/.config/lore/config.json` (or `$XDG_CONFIG_HOME/lo
 | `gitlab` | `baseUrl` | -- | GitLab instance URL (required) |
 | `gitlab` | `tokenEnvVar` | `GITLAB_TOKEN` | Environment variable containing API token |
 | `projects` | `path` | -- | Project path (e.g., `group/project`) |
+| *(top-level)* | `defaultProject` | none | Fallback project path used when `-p` is omitted. Must match a configured project path (exact or suffix). CLI `-p` always overrides. |
 | `sync` | `backfillDays` | `14` | Days to backfill on initial sync |
 | `sync` | `staleLockMinutes` | `10` | Minutes before sync lock considered stale |
 | `sync` | `heartbeatIntervalSeconds` | `30` | Frequency of lock heartbeat updates |
@@ -204,7 +206,7 @@ When showing a single issue (e.g., `lore issues 123`), output includes: title, d
 
 #### Project Resolution
 
-The `-p` / `--project` flag uses cascading match logic across all commands:
+When `-p` / `--project` is omitted, the `defaultProject` from config is used as a fallback. If neither is set, results span all configured projects. When a project is specified (via `-p` or config default), it uses cascading match logic across all commands:
 
 1. **Exact match**: `group/project`
 2. **Case-insensitive**: `Group/Project`
@@ -508,12 +510,15 @@ lore init --force            # Overwrite existing config
 lore init --non-interactive  # Fail if prompts needed
 ```
 
+When multiple projects are configured, `init` prompts whether to set a default project (used when `-p` is omitted). This can also be set via the `--default-project` flag.
+
 In robot mode, `init` supports non-interactive setup via flags:
 
 ```bash
 lore -J init --gitlab-url https://gitlab.com \
   --token-env-var GITLAB_TOKEN \
-  --projects "group/project,other/project"
+  --projects "group/project,other/project" \
+  --default-project group/project
 ```
 
 ### `lore auth`