cargo publish can't resolve cross-repo path dep (gitbot-shared-context, no version)

[hyperpolymath]

Surfaced by the estate latent-CI sweep. Cargo.toml has:

gitbot-shared-context = { path = "../../shared-context" }

a path-only dep with no version that points outside this repo (into the gitbot-fleet monorepo layout). cargo publish/cargo package reject path deps lacking a registry version, so the crates.io publish — and the cargo package step added for build-provenance in #94 — cannot succeed as-is. (Same class as filesoup hyperpolymath/filesoup#44.)

Decision needed: is echidnabot meant to publish to crates.io standalone?

• Yes → gitbot-shared-context must be published to crates.io and given a version here (or vendored into this repo).
• No (it only builds inside the gitbot-fleet monorepo / ships as a binary) → drop the cargo publish job; the release binaries are the distribution channel.

Related: the cross-repo path dep is existing monorepo debt (gitbot-fleet#18).