feat(mcp): add create_custom_attribute and delete_custom_attribute tools

[yakky]

Description

Adds two MCP tools to manage custom-attribute definitions:

• create_custom_attribute(entity_type, project, name, description="", type=None) – creates a definition for user stories, tasks, issues or epics and returns it (its id is the attribute_id for set_custom_attribute_value). type is omitted from the payload when not given. A duplicate name surfaces Taiga's real message.
• delete_custom_attribute(entity_type, project, attribute_id, expected_name) – deletes a definition. Destructive (Taiga also drops the stored values), so it refuses, deleting nothing, unless the attribute belongs to project and is named expected_name (case and surrounding spaces ignored). Both guards are needed: attribute ids are global across projects and numbered separately per entity type, so the same id can name a different attribute on another kind.

Both use the existing *Attributes factories; no library change. Needed by the Nephila taiga-sprint-planning skill to create an "Estimation" attribute on boards that lack one.

Stacked on #279 (bugfix/278-…): this branch builds on the MCP server code there, so this PR targets that branch and shows only its own 5 commits.

References

Taiga US 14050 (https://taiga.nephila.it/project/nephila/us/14050)

Checklist

• I have read the contribution guide
• Code lint checked via inv lint (ruff, isort, black, pypi-description: OK)
• changes file included (changes/14050.feature)
• Usage documentation added in case of new features (docs/mcp.rst)
• Tests added (unit tests for both tools + a list-tools test; 468 pass)

Verification

Live-tested on a Taiga instance, on a test board:

• create → duplicate name rejected with Taiga's message → delete with a wrong name refused → delete with the wrong entity type refused (an issue attribute with the same id exists on another project: the project guard stopped it) → delete with the right name and a different case OK → board read back unchanged.
• A cross-project delete was also refused before the name guard existed.

Note: the repo's pre-commit "ruff (legacy alias)" hook fails on ~99 pre-existing errors in untouched files; inv lint is clean and the new code adds none.

AI usage

Model: Claude Sonnet 5.5 (Claude Code), with Opus subagents as whole-branch reviewers.
Scope: design, tests-first implementation, docs and this PR text were drafted with AI; each commit and the push were reviewed and approved by the author. The create tool and the first version of the delete tool (fba0752) had an independent AI review; the later expected_name change (2307b6e) and the missing-owner test (cdfd016) were written in response to that review and have not had a second independent pass.
Prompts (summary): extend the MCP server so the planning skill can create a missing custom attribute; add a delete tool and live-test both on a test board; fix the review finding that an id can match a different attribute on another entity type.

Agent usage

• Design: brainstorming + plan writing, ~60k tokens, ~20m
• Implementation: inline plan execution incl. the delete tool and the name-guard fix, ~110k tokens, ~45m
• Review: two Opus whole-branch reviewers, ~70k and ~65k tokens, ~1m each; code-eval and doc-sync ~15k tokens, ~5m

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (6a1f9c5) to head (2307b6e).

Additional details and impacted files

@@                              Coverage Diff                               @@
##           bugfix/278-mcp-epic-link-and-version-fixes     #283      +/-   ##
==============================================================================
+ Coverage                                       98.22%   98.25%   +0.03%     
==============================================================================
  Files                                              12       12              
  Lines                                            1576     1607      +31     
  Branches                                          151      154       +3     
==============================================================================
+ Hits                                             1548     1579      +31     
  Misses                                             19       19              
  Partials                                            9        9              

Flag	Coverage Δ
unittests	98.25% <100.00%> (+0.03%)	⬆️

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[coveralls]

coverage: 98.818% (+0.02%) from 98.794% — feature/us-14050-mcp-create-custom-attribute into bugfix/278-mcp-epic-link-and-version-fixes