fix(security): resolve critical audit findings in marketplace and frontend

CRITICAL fixes:
- marketplace.rs: Replace fake SHA-256 (SipHash) with real sha2 crate
- marketplace.rs: Reject empty checksums (refuse unsigned plugins)
- marketplace.rs: Add install path traversal protection (reject ../|/|\)
- marketplace.rs: Add HTTPS-only URL validation on download_url
- marketplace.rs: Add curl --proto =https to block file:/gopher: SSRF
- marketplace.rs: Add --max-filesize 50MB download cap
- marketplace.rs: Add --no-same-owner --no-same-permissions to tar extraction
- marketplace.rs: Post-extraction path validation (canonicalize check)

Frontend fixes:
- pro-bridge.ts: Rename Symbol→CodeSymbol (avoid global collision)
- pro-bridge.ts: Tighten trust type to union 'human'|'agent'|'auto'
- PluginMarketplace.svelte: URL sanitization (reject non-https hrefs)

Remaining audit fixes (HIGH/MEDIUM/LOW) being applied by background
agents — will be committed separately when complete.

agor-pro/Cargo.toml

@@ -14,3 +14,4 @@ serde_json = "1.0"
 log = "0.4"
 dirs = "5"
 tokio = { version = "1", features = ["process"] }
+sha2 = "0.10"