Add --title flag and polish attestation rendering

[stevenolen]

Description

Three small improvements to ptd attestation driven by customer feedback on a generated document.

1. New --title flag overrides the rendered document title. Default behavior is unchanged: when the flag is not set, the title remains "Installation Confirmation — <target>". The flag is helpful when the internal workload name does not match the identifier a recipient expects at the top of the document.
2. Widen the Identity Provider column in the PDF auth table. The previous column widths [4, 3, 5] truncated long OIDC issuer URLs (e.g. Entra ID tenant URLs). Rebalanced to [3, 1, 8] since the Method column only renders short strings like oidc.
3. Surface the generation timestamp explicitly near the Sign-Off block in both the markdown and PDF renderers, and pre-fill the Prepared By date with the generation date. The date is still editable by anyone signing manually.

Code Flow

AttestationData gains a Title field and a DisplayTitle() method. The renderers (markdown.go, pdf.go) now call DisplayTitle() instead of inlining the default. The CLI sets data.Title from the new flag before invoking the renderers.

PdfSignatureRow now takes an optional date argument so the sign-off row can be pre-filled.

Category of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have reviewed my own diff and added inline comments on lines I want reviewers to focus on or that I am uncertain about

Verification

• just test-lib (attestation package tests pass)
• just cli builds cleanly
• ptd attestation --help shows the new flag with the documented default