fish: don't enable private mode when --no-config is set - #12741

Closes #12711.

fish --no-config -c 'set -g fish_history fish; history merge' errors with history: can't merge history in private mode. The reporter expects --no-config to skip default config dirs without touching history behaviour; --private already covers the no-history case.

The cause is in src/bin/fish.rs. The -N / --no-config arm sets both opts.no_config and opts.enable_private_mode:

'N' => {
    opts.no_config = true;
    // --no-config implies private mode, we won't be saving history
    opts.enable_private_mode = true;
}

The documented behaviour of --no-config (doc_src/cmds/fish.rst) is just "Do not read configuration files." There is no docs note that says --no-config should imply private mode, and --private is the documented way to opt into "fish will not access old or store new history."

The patch drops the implicit enable_private_mode = true. --private still triggers private mode on its own, and --no-config --private together still produce a private no-config shell.

Verification

Before:

$ ./fish --no-config -c 'set -g fish_history fish; history merge'
history: can't merge history in private mode

After:

$ ./fish --no-config -c 'set -g fish_history fish; history merge; echo OK:$status'
OK:0

$ ./fish --private -c 'set -g fish_history fish; history merge'
history: can't merge history in private mode

$ ./fish --no-config --private -c 'set -g fish_history fish; history merge'
history: can't merge history in private mode

Tests

Extended tests/checks/no-config.fish to assert fish_private_mode is unset under --no-config and that history merge succeeds.

Full local run: python3 tests/test_driver.py target/release -> 242 / 242 passed (6 skipped).

Changelog

Added a bullet under the unreleased section in CHANGELOG.rst.