TextField `variant: "obscured"` renders the value in cleartext

[joestump-agent]

Part of #54, but independent — this is a privacy bug, not a styling gap, and should ship first.

Problem

TextField.Variant is silently dropped by renderTextField (render/fields.go:51-75): the renderer reads label/value but never the variant. The basic catalog defines shortText / longText / number / obscured — and obscured is the password/secret case. Today a model that correctly asks for a secret with "variant": "obscured" gets the typed value echoed to the terminal in cleartext, both while editing and in the rendered value readout.

Fix

• When Variant == "obscured": echo the value as mask characters (• per rune) in both the display state and the editing state. The real value is still stored and still returned by FieldValues() — masking is display-only.
• Add a fixture + test pinning that the raw rendered output never contains the typed value for an obscured field.
• The other variants (number, longText, shortText) remain display-equivalent for now; document that in docs/wire-format.md (see the hint-audit docs issue).

Notes

Worth auditing FieldValues() consumers in hosts afterward: the value legitimately flows back to the agent on submission (that's the point of the field), but hosts may want to redact it in any human-readable submission prompt they synthesize.