feat: MODIFY STRUCTURE / ALTER TABLE — alter an existing table (#6)

[DDecoene]

Summary

Implements issue #6 — alter an existing table's columns without DROP+CREATE (and the data loss that entails).

• Command family (scriptable, usable from .prg):
  • ALTER TABLE <t> ADD <col> <type>
  • ALTER TABLE <t> DROP <col>
  • ALTER TABLE <t> RENAME <old> TO <new>
  • ALTER TABLE <t> ALTER <col> <type> (type change via rename→create→insert(CAST)→drop)
• MODIFY STRUCTURE opens an Assistant wizard (diff editor) for the table in USE; also reachable via the new sidebar "Modify structure…" action. Both paths funnel through the server (MODIFY_STRUCTURE action → modstruct-open WS message) so the wizard always gets authoritative column data.
• ADD/DROP/RENAME use native SQLite column ops; column ops that can invalidate an index drop the table's indexes (SQLite + metadata) and warn to rebuild with INDEX ON.

Version bumped to 0.7.0; CHANGELOG, README, and CLAUDE.md updated (roadmap item marked done).

Test Plan

• npm test — 188 vitest tests pass (new tests/AlterTable.test.ts: parser, ADD/DROP/RENAME/type-change round-trips, index-drop warnings, error cases, modstruct-open emission)
• npx playwright test tests/assistant.spec.ts — 10/10 pass (new wizard round-trip E2E)
• npm run build — typecheck clean