Move config-file creation and email prompt into code-trace setup

[winjer]

Stacked on #11 (base is the #11 branch). Merge #11 first; GitHub will then retarget this to main.

Second slice of the installer-slimming direction: config-file creation and the Langfuse user-id email prompt move out of install.sh into the binary as code-trace setup --write-config.

What changed

• src/setup.rs — pure, unit-tested helpers:
  • config_has_user_id — reuses the runtime config parser (config::parse_config_str), so a commented # LANGFUSE_USER_ID=… placeholder correctly does not count as configured.
  • decide_write / fresh_config / with_appended_user_id — the latter inserts a missing trailing newline so a hand-edited config is never concatenated (fixes the latent nit flagged back in Attach optional LANGFUSE_USER_ID to traces #8).
  • a write_config IO layer + terminal prompt. New flags: --write-config, --config-file, --user-email, --no-prompt.
• Prompt moved into the binary. Because the prompt now runs in a separate process reading /dev/tty, it can no longer consume the piped curl | bash script — the whole class of stdin-consumption bug we fixed in Read installer email prompt from /dev/tty so piped installs ask #9/Route OpenCode/Pi install prompts through /dev/tty too #10 is gone by construction.
• install.sh — create_config shrinks to a setup --write-config call plus the closing message. The OpenCode/Pi prompts remain in bash for now (next slice), so TTY_IN/resolve_tty_in stay.
• Tests — 10 new unit tests on the pure helpers + 5 integration tests driving the real binary (fresh+email, fresh placeholder, append, already-configured left untouched, existing no-op).

Verification

• cargo test (103 lib incl. 24 setup + 12 integration) and cargo clippy --all-targets clean.
• Verified end to end through a piped curl | bash simulation with OpenCode installed (real pipe for stdin, pty for /dev/tty): the binary prompts for and writes the email — the exact scenario reported broken earlier.

Next slice, when you want it: OpenCode/Pi plugin install (which would also let plugin install work under curl | bash, by embedding the plugin sources in the binary — today it only works from a local checkout).

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