Fix INK-9: bump Python floor to >=3.11 to match inkbox SDK

[dimavrem22]

Problem

codex-plugin advertised Python >=3.10 in three places, but its dependency inkbox>=0.4.24,<1.0.0 declares requires-python >=3.11. On Python 3.10, pip/uv pip install fails with unsatisfiable requirements — after install.sh has already accepted the interpreter, so a 3.10 user following the README hits a confusing mid-install failure.

Fix

Align every surface on >=3.11:

Surface	Before	After
pyproject.toml requires-python	>=3.10	>=3.11
install.sh finder	tries python3.10, gates min -ge 10	drops python3.10, gates min -ge 11
install.sh messaging	"Python 3.10+" / "No Python 3.10+ found"	"3.11+"
README.md	3× "3.10+"	3× "3.11+"
.github/workflows/tests.yml matrix	["3.10", "3.12"]	["3.11", "3.12"]

The CI matrix change is beyond the original issue's three-surface list but is the same drift: the unit lane was exercising a version the plugin can no longer support. (claude-code-plugin already declares >=3.11 and is unaffected.)

Now requires-python rejects 3.10 up front with a clear message instead of failing partway through dependency resolution.

Verification

• Reproduced the install.sh version gate for 3.9/3.10/3.11/3.12/3.13 → 3.10 now rejected, 3.11+ accepted.
• bash -n install.sh passes; grep confirms zero remaining 3.10 mentions in source.
• Confirmed inkbox 0.4.24 requires-python = ">=3.11" in inkbox/sdk/python/pyproject.toml.

Closes INK-9.