Feature: datetime → epoch collapsing for dicts/lists

[amahi2001]

Summary

Collapse ISO-8601 datetime strings to integer epoch values inside DictMinimizer and ListMinimizer. Currently only LogMinimizer strips timestamps (aggressive mode), and dict keys like timestamp/created_at get shortened to ts, but the values pass through untouched.

Motivation

ISO-8601 with timezone ("2025-04-17T21:05:33.123Z") burns ~13 tokens on cl100k. The epoch int (1760763933) burns 3-4. Multiplied across every row in a typical API response, this is one of the highest-ROI compressions available — and agents almost always want a number, not a human-readable string.

Reported by a user on Reddit as the single biggest win in their stack.

Proposed behavior

• Detect ISO-8601 values (date, datetime, with/without tz, with/without fractional seconds) in DictMinimizer and ListMinimizer
• Convert to int epoch seconds by default
• Use epoch milliseconds when sub-second precision is present in the source
• Opt-out flag for cases where the agent genuinely needs the string form
• Consider a companion epoch_to_iso reverse helper for downstream tools that want readable dates back

Open questions

• Assume UTC for naive datetimes, or skip conversion?
• Handle datetime.datetime / datetime.date Python objects in addition to strings?
• Should this be on by default or behind a flag? (Leaning: on by default — fits the zero-config ethos.)

Scope

• New: ISO-8601 detection regex (precompiled at module level, per project rules)
• Touch: src/ptk/minimizers/_dict.py, src/ptk/minimizers/_list.py
• Tests: unit coverage for tz-aware, tz-naive, fractional seconds, date-only, invalid strings (must pass through), and adversarial ReDoS cases
• Docs: README note + CHANGELOG entry