Scaffold arghda-core v0.1 with two linter rules

Rust crate parked at echo-types/arghda-core/ until it gets its
own repo. Implements the v0.1 surface from docs/arghda-spec.adoc:

- Workspace struct with four-state (inbox/working/proven/rejected)
  filesystem-backed layout; init() and open() are idempotent
- notify-based filesystem watcher helper
- LintRule trait + LintContext with include_root and entry_module
- missing-safe-pragma rule: scans the head of each file for both
  --safe and --without-K, emits hard-block if either is missing
- orphan-module rule: builds the transitive import closure starting
  from All.agda (tolerating stdlib modules as silent absent files),
  hard-blocks any .agda file outside that closure
- CLI (arghda): init, scan --json, watch
- Smoke test asserts the echo-types Agda suite passes both rules
  at the current commit (19 files, 0 hard-blocks)

https://claude.ai/code/session_01DDGQFBtAT9JdRwmu25QzC2

Author: claude

.gitignore

@@ -1 +1,2 @@
 *.agdai
+arghda-core/target/