deps: raise Ruby floor to >= 3.2 and test a real CI matrix (#15)

[minerva-sky]

Closes #15. Accepted lean from the 2026-09-10 contract-calls batch.

• agentic.gemspec: required_ruby_version >= 3.0.0 → >= 3.2. The old floor was never honest: locked async 2.24 / console / io-event already require 3.1, and 3.0/3.1 are EOL.
• .github/workflows/main.yml: matrix ['3.2', '3.3', '3.4', 'ruby'] with fail-fast: false. Minor-version strings let setup-ruby pick the latest patch instead of freezing on 3.2.4.
• CHANGELOG.md: entry under a new [Unreleased] heading.

Version number left alone on purpose. lib/agentic/version.rb says 0.2.0 while CHANGELOG.md already has a [0.3.0] - 2025-08-18 section, so I can't tell which number the "minor bump" lands as. Several accepted items (#10, #16, #20) are also minor-bump changes, so one release commit setting the number once seems cleaner than each PR guessing. Say the word and I'll bump here instead.

🤖 Generated with Claude Code

[minerva-sky]

Rebased onto current main (past #25/#26). Only conflict was CHANGELOG.md: both sides inserted a block above 0.2.0, so the Unreleased entry now sits above the 0.3.0 block. Diff is unchanged otherwise (CI matrix, gemspec floor, CHANGELOG line). Local note: on Ruby 4.0.2 the suite fails at load with require "benchmark" (spec/performance/component_performance_spec.rb) because benchmark is a bundled gem in 4.0 and is not in the Gemfile. That is pre-existing on main, not from this branch; CI's 3.2/3.3/3.4 matrix is the real check here.

[minerva-sky]

Pushed e0003e9: adds gem "benchmark" to the Gemfile. benchmark is a bundled gem since Ruby 4.0, so the head-Ruby job was failing at load on spec/performance/v0_3_0_benchmark_spec.rb. Other three Ruby jobs were already green.