feat(mcp): add include_user_stories option to list_milestones/get_milestone

[yakky]

Description

Adds an include_user_stories option (default True) to the MCP server's
list_milestones and get_milestone tools. Milestone.user_stories is always
fully expanded by python-taiga's parser into complete UserStory objects, which
the MCP server's to_jsonable() then serializes in full — making the embedded
field potentially large. Passing include_user_stories=False strips that key
from each returned milestone via a new _strip_user_stories() helper. Default
behavior (and output shape) is unchanged for existing callers.

Also updates docs/mcp.rst to document the new flag.

Builds on issue/mcp-optional-project-param (project-parameter optionality fix
for list_milestones/list_wiki_pages) — this PR is stacked on that branch.

References

No linked issue (minor/internal MCP server improvement).

Checklist

• I have read the contribution guide
• Code lint checked via inv lint
• changes file included — skipped: no GitHub issue number exists for this minor change, and changes/<issue>.<type> fragment names must reference one per CONTRIBUTING.rst
• Usage documentation added in case of new features
• Tests added

AI usage

Model: Claude Sonnet 5 (Claude Code), via the /nephila-flow company lifecycle command.

Scope: Full implementation — design discussion (bounded-path brainstorming),
TDD implementation, an independent code-review subagent pass, documentation sync,
and a rubric-scored evaluation (94%, artifacts/evaluations/2026-09-14-mcp-milestone-user-stories-toggle.md).

Prompts (faithful summary): "add to taiga.mcp_server.server.list_milestones
the option to return the full taiga response for the given endpoint or to remove
user_stories attribute from returned data" — refined through clarifying questions
into the include_user_stories toggle described above, then implemented and
reviewed per the standard flow, with the human author approving the design, the
branch, each commit, and the push at every gate.

Agent usage

Stage	Agent/skill	Tokens	Time
Review	superpowers:requesting-code-review (general-purpose subagent)	~70k	~60s

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.69%. Comparing base (b0043c2) to head (1547d81).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           issue/mcp-optional-project-param     #275      +/-   ##
====================================================================
+ Coverage                             97.68%   97.69%   +0.01%     
====================================================================
  Files                                    12       12              
  Lines                                  1340     1346       +6     
  Branches                                 94       95       +1     
====================================================================
+ Hits                                   1309     1315       +6     
  Misses                                   20       20              
  Partials                                 11       11              

Flag	Coverage Δ
unittests	97.69% <100.00%> (+0.01%)	⬆️

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[coveralls]

coverage: 98.514% (+0.007%) from 98.507% — issue/mcp-milestone-user-stories-toggle into issue/mcp-optional-project-param