feat: add type annotations to core and tasks modules

[Tokenized2027]

What

Type annotations added to the two foundational modules (core.py and tasks.py) plus a py.typed PEP 561 marker file.

core.py — 29 public functions annotated, including:

• Vault discovery: find_root, vault_path, tasks_dir_path
• Frontmatter parsing: parse_frontmatter, parse_multiline_value, parse_inline_list, parse_frontmatter_list, parse_frontmatter_for_json, extract_body, strip_wikilink
• Text utilities: slugify, title_from_description, shlex_join
• Output helpers: info, err, warn, header, step, detail, success, failure, hint, remaining_line, format_duration, tips_block, summary_block
• Module state setters: set_quiet, set_run_start

tasks.py — 13 public functions annotated, including:

• discover_tasks, resolve_task, group_tasks_for_display, format_table
• validate_transition, allowed_from
• update_frontmatter, next_task_id, create_task_file
• All four command handlers: cmd_list, cmd_show, cmd_create, cmd_status

Why

• IDE autocompletion and inline documentation for callers of these modules
• Type checkers (mypy, pyright) can now catch misuse at the call sites
• Makes the public API self-documenting — resolve_task returning tuple[str | None, str | None, int] is immediately clear to a reader

Scope and approach

• from __future__ import annotations at the top of each file — clean Python 3.9+ compatible syntax with no runtime cost
• from typing import Any, Optional only where needed (union types use X | Y syntax via the future import)
• Only function signatures annotated — zero logic changes, zero new dependencies, zero docstrings added or modified
• Private helpers (_check_dir, _git_toplevel, _use_color, etc.) intentionally left unannotated
• py.typed empty marker file signals to PEP 561-aware type checkers that this package supports typing

Tests

All existing tests continue to pass. The annotation-only changes introduce no behavioural differences.