fix: resolve session keys before destroying

[zakazaka95]

Summary

• resolve the requested identifier against stored sessions case-insensitively
• filter by --mode when provided and delete using the canonical stored session key
• cover mixed-case identifiers with both inferred and explicit session modes

Fixes #396

How did you test your changes?

• account-cli typecheck and build
• Biome lint and format checks on the changed files
• bounded compiled-CLI smoke tests for both destroy paths
• core session unit suite: 53/59 passed; the remaining six are unchanged Windows permission-mode failures

The repository's integration harness stops before CLI execution on this Windows environment because its existing execFileSync('npx', ...) call returns spawnSync npx ENOENT.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1