Add next_step to diagnostics and refine worker comments

- Add `next_step` field to `Diagnostic` with field-specific operator guidance; `doctor` prints it for every finding
- Replace emoji-decorated familiar comments with plain, actionable copy linking directly to the session URL
- Extract `cave_session_url`, `starting_comment`, and `pr_opened_comment` helpers with unit tests
- Upgrade tower-http from 0.5 to 0.6 and drop unused deps (octocrab, arc-swap, async-trait, serde_json from config, etc.)
- Fix `cave_base_url` example config to not point at the webhook server
- Update README, self-hosting docs, and starter example accordingly

Author: BunsDev

.gitignore

@@ -3,3 +3,4 @@ config/local.toml
 keys/
 *.pem
 .env
+.DS_Store

Cargo.lock

@@ -30,7 +30,7 @@ serde_json = "1"
 sha2 = "0.10"
 tokio = { version = "1", features = ["full"] }
 tower = "0.5"
-tower-http = { version = "0.5", features = ["trace"] }
+tower-http = { version = "0.6", features = ["trace"] }
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 uuid = { version = "1", features = ["v4"] }