docs: document `cora upgrade` in cli-reference (self-upgrade is undocumented)

[ajianaz]

What

cora upgrade (self-upgrade) is implemented and wired (src/commands/upgrade.rs, main.rs) but missing from docs/cli-reference.md and the README install/upgrade section.

Proposal

Add to docs/cli-reference.md:

• `cora upgrade --check` — check for a newer release, no download
• `cora upgrade [-y]` — detect OS/arch, download the latest GitHub release asset, verify SHA-256 checksum, replace the running binary
• Note the background update notification: non-blocking check on startup, 24 h cache at ~/.codecora/cora-code/update-cache.json, stderr-only, disabled via --no-update-check
• Note for cargo-from-source users: if the running binary came from cargo install --path ., prefer re-running that instead

Impact

Users (and agents) currently discover self-upgrade only by reading help output. Verified working against v0.14.0: cora upgrade --check correctly reports current 0.13.0 vs latest v0.14.0 with release notes link. Fits the next docs pass.