Release v0.4.0 — zero-config first run

[abhiksark]

Release v0.4.0 — zero-config first run

Promotes the zero-config onboarding work to main for tagging and release.

What's new

• Zero-config first run: pythonlings with no workspace now creates one at ~/.pythonlings and opens the first exercise — no init, no --path, no cd. (Fixes the sharp edge where pythonlings init defaulted to the current directory and failed inside a non-empty folder.)
• PYTHONLINGS_HOME env var to relocate the default workspace; --path/--root still work for explicit locations.
• Friendlier init: idempotent on an existing workspace (a note instead of an error), and an actionable message for a non-empty, non-workspace directory.
• Every command resolves its workspace the same way: explicit --root → current dir if it's a workspace → ~/.pythonlings.

Implementation

New core/workspace.py (default location + precedence resolver); CLI wiring so only TUI-launch commands auto-create; version bumped to 0.4.0. Built via brainstorm → spec → plan → subagent-driven implementation, each task spec+quality reviewed, plus a clean final whole-branch review.

Verification

• python -m pytest -q → 136 passed
• Built wheel installs and reports pythonlings 0.4.0
• Workspace smoke (init/list/solution/reset/verify) passes

Closes the onboarding UX issue raised in this cycle.