refactor(cli): flat migrations layout + filename-encoded plugin attribution

[dnahilman]

Closes #32.

Summary

Migration files now live in a single flat directory with the plugin name encoded in each filename. This matches the conventions of golang-migrate, goose, atlas, Rails, and Prisma.

Before (nested):

.
├── migrations/20260520120000_initial.up.sql
└── plugins/username/migrations/20260520130000_add_username.up.sql

After (flat):

.
└── migrations/
    ├── 20260520120000_core_initial.up.sql
    └── 20260520130000_username_add_username.up.sql

Breaking change (pre-v1)

• Source migration files in the repo were renamed to the new convention. Applied-migration tracking is unaffected (tracking table keys on the 14-digit timestamp ID, which is unchanged).
• migrations.plugins[] in goten.config.yaml is now strictly a list of plugin shorthand names (- username). Explicit-path entries (- ./plugins/username/migrations) are no longer accepted.
• The legacy nested layout is not auto-detected; existing projects must regenerate their migrations directory via goten init (or rename manually).

What's in the PR

• R1 Renamed source SQL files: core (*_core_*) + username plugin (*_username_*).
• R2-R3 discovery.go: walks only cfg.Migrations.CoreDir; parses plugin from filename via regex ^(\d{14})_([a-z][a-z0-9]*)_(.+)\.up\.sql$. Files that don't match the pattern are skipped.
• R4 init.go: writes both core and plugin SQL files to cfg.Migrations.CoreDir. No more per-plugin destination subdirs.
• R5 JSON Schema (goten.config.schema.json): migrations.plugins[] items now restricted by regex to lowercase shorthand names.
• R6-R7 New cmd/goten/scan.go: parses user's *.go files (parser.ImportsOnly), finds imports under github.com/dnahilman/goten/plugins/, warns when those drift from migrations.plugins. Skipped dirs: vendor, .git, .claude, node_modules, testdata. Honors blank imports. Opt out with --no-scan.
• R8 Tests: updated existing for new filenames, added 6 new scan tests + 1 filename-skip test. Removed resolvePluginEntry and its test (function dropped — no longer needed without path form).
• R9 examples/basic/goten.config.yaml switched to shorthand. examples/basic/migrations/ is now .gitignored and regenerated by make example via goten init. Integration test (test/adapters/gorm/adapter_test.go) updated to read the renamed core file.
• R10 CHANGELOG [Unreleased] Breaking-change section + Added section.

Test plan

• make build — all six modules compile.
• go test ./internal/... — unit tests pass.
• cd test && go test ./... — handlers/session/crypto + adapters/gorm/integration + plugins/username all pass with renamed files.
• cd cmd/goten && go test ./... — 18 tests pass (init + discovery + scan + config).
• Manual smoke: mktemp -d → echo config with plugins: [username] → goten init --no-scan → confirm 4 files in flat ./migrations/, no ./plugins/ directory created. Output:

  core:        2 written, 0 skipped, 0 overwrote
  username:    2 written, 0 skipped, 0 overwrote
  

• Reviewer: spot-check that the integration test still finds the renamed core SQL at test/adapters/gorm/adapter_test.go:32.

Follow-up

Wiki Quick Start §5 diagram update is the last step of #32 and will land as a wiki commit immediately after this PR merges.

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