organic-os v0.5.3 - the link renderer can no longer be steered

One security fix, found by reviewing a community PR that was itself safe.

The Monday report's link renderer emitted href values with attribute escaping disabled and no scheme check. Report content includes brain data - titles, reasoning, URLs - so a crafted link could break out of the attribute (a double quote followed by an injected handler) or ship a clickable javascript: or data: link into HTML that goes to Telegram, email, and PDF.

Now: URLs are attribute-escaped, and schemes are allowlisted - http, https, mailto, and relative paths render as links; anything else renders its label as plain text with the URL dropped. The check normalizes case and strips control characters first, so JaVaScRiPt: and tab-prefixed tricks do not slip past. Six new tests pin the escaped output itself, mutation-verified against the unfixed renderer.

Closes #19. Found during the review of #18 (which was clean - this defect predates it and sat in a line that PR never touched). The remaining sanitizer-completeness follow-up is tracked in #20.

428 tests, 9 audit checks, 8 gate probes, CI green.