fix: use host launch identity proof

[yehyakin]

What changed

• Move exact custom-role model and reasoning proof to the authoritative Host/tool mapping plus the parent launch record.
• Restrict the child handshake to observable permissions, operational constraints, and zero side effects.
• Preserve Fail Closed behavior and require Host-owned no-write verification when Sol inherits broader technical access.
• Add regression assertions for the corrected proof boundary.

Root cause

The v0.4.0 handshake required a child agent to self-report its actual runtime model and reasoning effort. On the current Desktop collaboration surface those values are fixed by the Host tool contract but are not introspectable by the child, so a correctly launched Sol always returned unproven and the workflow deadlocked before planning.

Impact

Sol Control can now pass its identity gate on supported surfaces without trusting an invented child self-report. Unsupported or ambiguous role mappings still fail closed.

Validation

• git diff --check
• bash scripts/validate.sh — PASS; PowerShell unavailable locally, deterministic structural checks used
• bash scripts/test.sh -v — 113 tests, OK
• bash scripts/install.sh --check — installation consistent, no changes required
• Fresh two-turn Sol forward test — handshake PASS, zero writes, final Sol review PASS