Add an agent skill for driving the proxy

[winebarrel]

The proxy leaves the agent two things it cannot work out from the tool schemas:

• Which profile to pass. Names are deliberately absent from the schemas, since list_profiles rereads ~/.aws/config on every call. An agent that does not list them first either guesses or fails.
• What a denial means. An AccessDenied under --sso-role is a decision. Left unsaid, an agent treats it as an obstacle and looks for another profile, another role, or the aws CLI — which carries no profile, is not signed by the proxy, and drops the narrowing entirely.

skills/awsmcproxy/SKILL.md states both, plus the SSO-login case that cannot be completed on the user's behalf and the absence of a per-call region argument.

It sits at skills/ rather than .claude/skills/ so it is installable by any agent:

npx skills add winebarrel/awsmcproxy          # into ./.claude/skills
npx skills add winebarrel/awsmcproxy -g       # into ~/.claude/skills

Verified with npx skills add . -l, which discovers the skill and prints its description.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.40%. Comparing base (65e4c84) to head (1231b05).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #12   +/-   ##
=======================================
  Coverage   98.40%   98.40%           
=======================================
  Files           5        5           
  Lines         377      377           
=======================================
  Hits          371      371           
  Misses          3        3           
  Partials        3        3           

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