Make tasks server the primary master plans doc surface deprecating MkDocs

Problem

Two doc servers confuse contributors. Once the tasks server covers browsing + live status, MkDocs should become secondary or retired for master plans.

Longer-term end state (tracked across follow-on tasks): just tasks serve as the only local “plans site,” with navigation that mirrors milestones → workstreams → tasks/initiatives on the server, and eventual removal of just plans once parity and static export (if any) are settled.

Context

• Dependency add-tasks-serve-command-and-plans-server is completed (PR #712 merged). The tasks server ships /status, /status.json, and /browse/ endpoints via FastAPI on port 8005.
• MkDocs (just plans, port 8004) remains active for: Material theme search, macro-rendered content (macros.py), Mermaid diagrams, and the static deploy pipeline (.github/workflows/deploy-static-sites.yml → GCS bucket).
• CI still runs mkdocs build -f tasks/mkdocs.yml for static hosting. Removing this requires a separate static-export story for the tasks server.
• Operator /status UX: improve-tasks-serve-status-ui-for-operator-triage.
• Rename + paths: rename-master-plans-tree-to-tasks — committed tree → tasks/, runtime state → <repo>/.tasks/, explicit TASKS_ROOT / --tasks-root instead of hard-coded tasks/ path math.

Possible Solutions

• A — Docs-only migration: Update AGENTS.md, cli-reference.md, and justfile help to prefer just tasks serve for local browsing. Keep MkDocs for static deploy and as a secondary option. No code changes, no CI changes.
• B — Remove MkDocs for plans: Delete tasks/mkdocs.yml and all MkDocs-specific assets after building a static export path for the tasks server. Requires CI pipeline changes.
• Recommended: A now. B only after a static-export story ships and stakeholder sign-off.

Plan

• [x] Audit feature gap: tasks server covers live status + markdown browse; MkDocs adds search, macros, Mermaid, and static deploy. Gap accepted for local dev workflow — search and macros are not needed for day-to-day task browsing.
• [x] Update AGENTS.md to list just tasks serve as primary local workflow, MkDocs as secondary/static.
• [x] Update tasks/tools/cli-reference.md to reflect primary status.
• [x] Update justfile help text to label tasks server as primary and MkDocs as legacy.
• [ ] (Future — out of scope) Adjust CI/publish pipeline once static export story exists.
• [ ] (Future) README/CLAUDE entrypoints still to prefer just tasks serve explicitly (see Implementation Progress).

Implementation Progress

Audit: feature gap accepted

For local dev and operator workflows, live task status is the primary need. Search and macros are nice-to-haves used mainly for milestone/strategy docs, not daily task work. Gap accepted.

Changes made

• AGENTS.md: Updated "Plans serving modes" to prefer just tasks serve as the primary local workflow, with just plans as secondary for static content.
• cli-reference.md: Removed "separate from MkDocs" framing; stated tasks server is the primary local surface.
• justfile: Updated help text to label tasks server as "Primary" and MkDocs as "Legacy".

Follow-up

• README.md / CLAUDE.md still do not consistently list just tasks serve as the primary operator path for master plans — track as a small docs sweep or reopen scope.

QA Exploration

• [x] N/A — docs-only changes, no UI affected

Review Feedback

• [ ] Review cleared