Adds the pdo/with-transaction macro so callers stop hand-writing the begin/try/commit/rollback dance:

(pdo/with-transaction conn
  (pdo/insert conn :accounts {:name "a" :balance 100})
  (pdo/insert conn :accounts {:name "b" :balance 0}))
;; commits + returns the last body value, or rolls back + re-throws on error

Semantics (per #20):

• (begin conn) before the body; (commit conn) on normal completion; macro evaluates to the last body form.
• On any \Throwable: (rollback conn) then re-throw the original.
• Nesting: if (in-transaction conn) is already true, the body runs inline - no nested begin/commit (v1, no savepoints; documented).

Hygiene

conn is evaluated once into an auto-gensym conn#; result# / e# are gensym'd. Expansion calls begin/commit/rollback/in-transaction via syntax-quote qualification, so it resolves correctly from any caller namespace (proven by the tests living in phel.pdo-test).

• Tests: commit-path + return value, rollback-on-throw + re-throw, and nested-inline (inner call leaves the outer transaction open; outer rollback reverts the inline write).
• README transactions row + docs/recipes.md transactions section + CHANGELOG updated.

Polish pass: no refactor needed - single let/if/try form, no duplication.

composer test green (56 assertions).

Closes #20