Harden --migration_table_name: validation, naming consistency, and truncate support

[sinmetal]

WHY

#152 added the --migration-table-name flag. It works, but it left three issues behind.

1. The table name is interpolated into SQL/DDL without any validation.
   GetSchemaMigrationVersion builds "SELECT Version, Dirty FROM " + tableName + " LIMIT 1"
   and EnsureMigrationTable builds fmt.Sprintf("CREATE TABLE %s ..."). Until Add --migration-table-name flag to migrate command #152 the
   name was a compile-time constant, so this was safe. Now it is user input, and a value
   passed from a CI variable can change the statement. Passing an empty string also produced
   a confusing CREATE TABLE ( error from Cloud Spanner instead of a clear message.

2. The flag name does not follow the existing convention.
   Every other multi-word flag in wrench uses snake_case: credentials_file, schema_file,
   proto_descriptor_file. Only this one was kebab-case.

3. truncate silently destroys the migration version when a custom table is used.
   TruncateAllTables skips the literal SchemaMigrations table so that a truncated
   database keeps its migration version. With a custom table name, that exclusion does not
   match, the version row is deleted, and the next migrate up re-applies every migration
   from version 0 — failing with errors like Duplicate column name ... for DDL migrations.
   This is easy to hit because truncate is a routine command in development and CI, and
   the failure surfaces later in an unrelated command.

WHAT

Validate the table name and quote it (507e775)

• Validate against ^[A-Za-z][A-Za-z0-9_]{0,127}$ in getMigrationTableName, before
  connecting to Cloud Spanner, matching the Cloud Spanner table name rules
• Fall back to the default SchemaMigrations when the flag is empty, consistently with
  schemaFilePath()
• Quote the table name with backticks in GetSchemaMigrationVersion and
  EnsureMigrationTable, as TruncateAllTables already does
• Resolve the table name once per command instead of on every use

Rename the flag to --migration_table_name (6ec6969)

• The flag is not included in any release yet (latest tag is v1.13.5), so it is renamed
  without keeping an alias

Keep the specified migration table on truncate (6337f22)

• TruncateAllTables now takes the migration table name and excludes that table
• truncate accepts --migration_table_name
• getMigrationTableName moved to cmd/cmd.go since it is no longer migrate specific

Docs are updated: the README describes the name restriction, and notes that the same
--migration_table_name value must be given to migrate up / version / set and
truncate.

Testing

• TestGetMigrationTableName covers the empty fallback, a leading digit, an invalid
  character, a backtick-based injection string, and a name longer than 128 characters
• TestTruncateAllTables is new and covers both the default and a custom table name.
  Verified that it fails against the previous hardcoded exclusion
• Full suite passes against the Spanner emulator
• Manually verified against the emulator: truncate --migration_table_name Data_Migrations
  keeps the version, while truncate without the flag drops it to No migrations.ion why you submit this pull request
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[vvakame]

LGTM.

Note: This changes contains breaking change.

[knwoop]

@sinmetal Sorry for the late look — catching up after the merge. Thanks for the fix!