cli-parser: return empty on socket stdin to unblock Claude Code Bash tool

[SevenX77]

Problem

ccb ask (and other commands that call _read_optional_stdin) hangs indefinitely when invoked from Claude Code's Bash tool, because the tool passes a Unix-domain socket as child stdin:

1. sys.stdin.isatty() returns False (socket is not a TTY)
2. _read_optional_stdin unconditionally falls into read_stdin_text(), which blocks on read() until EOF
3. Claude Code's Bash tool never closes its end of the socket until the child exits → mutual wait → 100% hang

The workaround users have been living with is appending < /dev/null to every ccb ask invocation from Bash tool contexts. This is easy to forget and propagates through docs and agent scripts.

Fix

Add a stat.S_ISSOCK early-exit check using os.fstat(0) before entering read_stdin_text(). Other stdin types (TTY, pipe/FIFO, regular file) keep their original behavior. On fstat OSError, fall through to read_stdin_text() so the original code path is preserved (no regression when fd 0 is closed).

Code change: +8 lines in lib/cli/parser.py, one function touched.

Why S_ISSOCK?

Only fstat + S_ISSOCK can distinguish a Unix socket from a FIFO/pipe; fcntl(F_GETFL) exposes only flags (O_NONBLOCK etc.), not the underlying file type. Pipe/FIFO stdin (e.g. cat file | ccb ask ...) must continue to be read, so a coarser check (e.g. "any non-TTY non-regular-file") would break valid usage.

Tests

5 unit tests in test/test_cli_parser.py covering:

• Socket stdin → returns empty without calling read_stdin_text
• FIFO stdin → reads normally
• Regular file stdin → reads normally
• TTY stdin → returns empty without calling fstat (priority check)
• fstat raising OSError → falls back to read_stdin_text (no regression)

All 5 pass locally. E2E verified from Claude Code Bash tool: ccb ask --wait a2 "ping" (no < /dev/null) returns a normal reply in 15s instead of hanging.

Risk

Minimal. The only scenario that behaves differently is stdin-is-Unix-socket — previously that hung forever, now it returns '' (treated as "no stdin content supplied"). No known legitimate use case where ccb ask is fed meaningful data via a Unix socket stdin; the normal transports (pipe, file redirect, interactive TTY) are unaffected.

Rollback

Single commit, revert with git revert if needed.