[cleanup] Phase 2b: Exclude unused haystack providers from workspace builds

[AlexMikhalev]

Summary

Exclude haystack_atlassian, haystack_discourse, haystack_jmap, haystack_grepapp from workspace builds.

Actions

• Add haystack crates to Cargo.toml exclude list
• Keep directories intact for future use
• Clean commented-out haystack deps in terraphim_middleware/Cargo.toml

Verification

• cargo check --workspace passes
• cargo test --workspace passes

[AlexMikhalev]

PR #631 addresses this. Excluded haystack_atlassian, haystack_discourse, haystack_jmap, haystack_grepapp from workspace builds.