[python] Add Python 3.14 to the CI matrix once the dev/adapter ecosystem supports it

Context

Follow-up from #339 / PR #358. We expanded supported Python to 3.10–3.13 and made every matrix cell gating. 3.14 was deliberately deferred, not dropped on a whim:

contextweaver's own core dependencies already support 3.14 (CI pulled tiktoken/PyYAML cp314 wheels fine). The blocker is the heavy dev/adapter stack used by the test extras — all currently declare Requires-Python <3.14, so pip install -e ".[dev,langchain]" cannot resolve on Python 3.14.5:

• crewai
• mem0ai
• fastmcp
• langgraph
• langchain-core

(Observed on the first CI run of #358: ERROR: Could not find a version that satisfies the requirement crewai>=0.80 on 3.14.5, with every candidate Ignored ... Requires-Python <3.14.)

Done means

• Re-verify the above upstreams ship releases without the <3.14 cap (check periodically).
• Add "3.14" back to the test matrix in .github/workflows/ci.yml (gating).
• Re-add the Programming Language :: Python :: 3.14 trove classifier in pyproject.toml.
• Bump .github/workflows/deps-latest-weekly.yml from 3.13 → 3.14.
• Update the README/AGENTS Python-support statements to 3.10–3.14.
• If only some upstreams support 3.14, consider a reduced 3.14 install (core + lean test deps, skipping the lagging adapters) — but that also needs the ~5 test modules that import adapter deps at module scope to guard via importorskip so they skip cleanly. Decide gating vs allow-failure at that point.

Notes

No code change is blocked by this; it is purely a CI-matrix/metadata expansion gated on upstream availability. Related: #339, #356, #358.

First verify whether crewai, mem0ai, fastmcp, langgraph, and langchain-core have released versions without the Python 3.14 cap. Then inspect .github/workflows/ci.yml, .github/workflows/deps-latest-weekly.yml, pyproject.toml, and the README/AGENTS support statements; done means updating them for 3.14, or documenting the reduced-install and importorskip approach if upstream support remains partial.