Rename master_plans directory to tasks and align CLI paths

Problem

The repository and CLI should use one name (tasks) for the planning system; master_plans/ is legacy path noise.

Context

• Dependency task migrate-master-plans-docs-primary-to-tasks-server is completed — just tasks serve replaces MkDocs.
• 107 files referenced master_plans (underscore), 8 used master-plans (hyphen), 14 used MASTER_PLANS (uppercase).
• Committed content lived under master_plans/ as a Python package plus tools/ and content dirs.
• pyproject.toml registered plans console script → master_plans.tools.plans_cli:main.
• Runtime logs lived in master_plans/logs/ (gitignored). Moved to .tasks/ at repo root.

Possible Solutions

• Single big-bang rename PR — git mv + mechanical replacement across all files. Clean, no compat shims. Recommended given pre-launch no-backwards-compat policy.
• Symlink bridge — unnecessary complexity pre-launch.

Plan

• [x] git mv master_plans tasks — rename the directory
• [x] Update pyproject.toml — package name, console script entrypoint
• [x] Update justfile — all targets, PYTHONPATH, paths
• [x] Update .gitignore — replace master_plans/logs/* with .tasks/, update _site_master_plans
• [x] Create .tasks/ runtime state directory (gitignored)
• [x] Update Python imports in tasks/tools/*.py, tasks/activity.py, scripts/task_manager_lib.py, scripts/pr_body_lib.py
• [x] Update shell scripts — scripts/dispatch, scripts/pr-create, scripts/review, scripts/task-manager-heartbeat
• [x] Update test files — path strings, imports, fixtures; rename test files
• [x] Update CLAUDE.md, AGENTS.md, architecture docs
• [x] Update .claude/ and .codex/ commands and skills
• [x] Update CI/GitHub workflows and PR templates
• [x] Update ai_notes/ and remaining references
• [x] Global rg master_plans verification — zero remaining references
• [x] just ci passes (3 pre-existing failures unrelated to rename)

Implementation Progress

Approach

Single big-bang rename: git mv master_plans tasks followed by mechanical replacement of all master_plans references. No symlinks, no backwards compat (pre-launch policy). Runtime state moves from master_plans/logs/ to .tasks/.

Key changes

• Directory: master_plans/ → tasks/
• Runtime state: master_plans/logs/ → .tasks/ (gitignored at repo root)
• Python variable: MASTER_PLANS → TASKS_ROOT in plans_cli.py, task_manager_lib.py, tasks_server.py
• Env var: MASTER_PLANS_BUCKET → TASKS_BUCKET, MASTER_PLANS_COMPLETED_BY → TASKS_COMPLETED_BY
• CSS/JS: master_plans.css → tasks.css, master_plans.js → tasks.js
• Test files renamed: test_master_plans_activity.py → test_tasks_activity.py, etc.
• ~100 files updated across source, tests, config, docs, agent commands, skills, CI, ai_notes
• 3 pre-existing test failures confirmed on main — not introduced by this rename

QA Exploration

N/A — non-UI infrastructure rename task.

Review Feedback

• [ ] Review cleared