fix repl-block, check-tangle, commands: READMEs, exit codes, path resolution

repl-block:
- Listing mode exits 0, not 1 (listing is not an error)
- Dead lines parameter removed from find_by_function
- Block listing goes to stderr (not stdout) so piped use works
- Added README.org

check-tangle:
- Fixed tangle tool resolution: prefer local projects/tangle-tool/tangle
- Fixed path resolution for relative and absolute tangle paths
- Removed 2>/dev/null suppression so tangle errors are visible
- Added README.org

Commands:
- Rewrote all three .opencode/commands/*.md with proper prompts

.opencode/commands/check-parens.md

@@ -3,6 +3,9 @@ description: Check paren balance in lisp blocks of .org files
 ---
 
 Run `projects/check-parens/check-parens` on the given .org files to verify all
-`#+begin_src lisp` blocks have balanced parentheses.
+`#+begin_src lisp` blocks have balanced parentheses. Uses SBCL's reader for
+100% accuracy — no false positives from string literals or character literals.
+
+Exit 0 if all blocks balanced, 1 if any issues found.
 
 Usage: /check-parens <file.org> [<file.org> ...]