fix(word-count): treat apostrophes as part of a word

[MsfPablo]

Summary

Fixes #324. WORD_COUNT_REGEX didn't include the apostrophe character, so contractions like "can't" were matched as two separate words ("can" and "t") instead of one, inflating the word count.

• Add ' alongside the existing \- in the word-character class (the "Option A" fix suggested in the issue), so contractions and possessives count as a single word — consistent with how hyphenated words are already handled by the same regex.

Test plan

Verified the regex change against a handful of cases (contractions, possessives, hyphenated words, numbers with commas/decimals) — all count as expected, no regressions:

Input	Before	After
Can't	2	1
I can't won't hasn't	7	4
It's Sarah's book	5	3
This is a test	4	4
well-known fact	2	2
There are 1,000 words and 3.14 pi	7	7

I initially added a unit test under test/model/, but writing-session-tracker.ts imports from obsidian (a types-only package with no runtime entry), which vitest can't resolve outside the Obsidian runtime — the same reason the existing test/compile/steps/remove-links.test.ts only covers files that don't import obsidian. Happy to add a test if you'd like the word-counting logic split out into a pure module that could import cleanly.

[b-camphart]

FYI #338

[MsfPablo]

Closing in favor of #338, which fixes this same issue more completely (also catches a second real bug in the HTML-comment-stripping order) and has proper test coverage via a mocked obsidian module — something I wasn't able to get working in my own test setup. Thanks for the more thorough fix!