Tasks server Jinja macro expansion for plans markdown

Problem

tasks/*.md still contained {{ current_milestone_card() }} and similar MkDocs macro calls; the tasks server showed them literally after MkDocs removal.

Context

• tasks/macros.py defines define_env for mkdocs-macros-plugin; implementations use Path(__file__).parent as the plans tree root.
• tasks/mkdocs.yml extra.current_milestone drives rollup scope.

Possible Solutions

• A — Full Jinja templates per page: heavy.
• B — Jinja pass on markdown body, then Markdown (recommended): reuse define_env + macro callables; trust_html only on /tasks/* doc route.

Plan

1. tasks/tools/tasks_plans_jinja.py — load extra, TasksMacroEnv, expand_tasks_markdown_macros.
2. tasks_server.tasks_page — expand then _markdown_body_to_html(..., trust_html=True).
3. Tests: README integration + undefined macro.

Implementation Progress

• Added tasks/tools/tasks_plans_jinja.py (TasksMacroEnv, load_mkdocs_extra, expand_tasks_markdown_macros via Jinja2 StrictUndefined).
• tasks_server: _markdown_body_to_html(..., trust_html=); /tasks/{path} runs macro expansion before markdown.
• Browse markdown unchanged (still escaped; no macro expansion).
• Tests: tests/core/test_tasks_plans_jinja.py, test_tasks_readme_expands_macros_in_shell in test_tasks_server.py.

QA Exploration

• N/A — covered by unit/integration tests; / (plans hub) and legacy URL redirect HTML assertions.

• [x] QA exploration completed (or N/A for non-UI tasks)

Review Feedback

• [ ] Review cleared