Add nox session to install PTB agent skill from wheel

[jana-selva]

Summary

Add a PTB nox session that installs the packaged PTB agent skill from the installed PTB wheel into a project local agent skill directory.

Background

Issue #933 adds the PTB skill for agents. The skill is version dependent because it describes PTB sessions, PTB workflow behavior, release behavior, and configuration rules.

PR #937 packages the skill with PTB, but it does not yet provide the install or copy mechanism for projects. This follow-up issue tracks that missing part.

The preferred design from #933 is:

• package the skill as a PTB resource in the wheel
• provide a nox session that copies the skill into the project skill directory

This keeps the installed skill aligned with the PTB version used by the project.

Scope

Add a nox session or equivalent PTB task that:

• locates the packaged skill resource in the installed PTB package
• copies exasol/toolbox/skills/exasol-python-toolbox/ into the project agent skill directory
• keeps SKILL.md and all referenced files together
• creates the target directory if necessary
• overwrites or updates an existing installed copy in a predictable way
• reports what was copied and where

Open design points

• Decide the final nox session name.
• Decide the default project agent skill directory.
• Decide whether the target directory can be configured through PROJECT_CONFIG.
• Decide whether the session should support dry-run or force options.

Acceptance criteria

• A project that depends on PTB can install the PTB skill through the new nox session.
• The installed skill comes from the PTB package resources, not from a separately maintained tar file.
• The copied skill contains SKILL.md and all required reference files.
• Tests verify that the packaged skill can be found and copied.
• Documentation explains how to run the nox session.

Related work

• Follow-up to Provide a skill how to use the PTB for agents #933
• Related to PR #933 : agents skill - PTB #937