add custom prompts with PS1 and PS2

PS1 and PS2 support the backslash escapes of bash (\u \h \w \W \$ \t \j
\e \[ \] ...) and expand $VAR, ${...}, $(...) and $((...)) before each
prompt. Values of escapes and expansions are sanitized. Lines before the
last one and non-SGR escape sequences are written before readline's
prompt. Without PS1, the default prompt is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packaging/man/gosh.1

@@ -445,7 +445,9 @@ and 2 are supported.
 .PP
 Control structures may span several lines; an incomplete command shows
 the continuation prompt
-.BR "> " .
+.B $PS2
+(default
+.BR "> " ).
 .B !
 negates the exit status of a command.
 .B for
@@ -731,7 +733,9 @@ Exit the shell.
 .PP
 Lines starting with a space are not saved in the history.
 .PP
-The prompt shows
+If
+.B PS1
+is not set, the prompt shows
 .IR user @ host : dir
 followed by
 .BR $ .
@@ -746,6 +750,55 @@ the OSC 7 escape sequence
 so terminals like GNOME Terminal open new tabs and windows in the same
 directory. Nothing is sent if the output is not a terminal or
 .BR TERM=dumb .
+.SS Custom prompts
+.B PS1
+and
+.B PS2
+are expanded before each prompt. They may contain the backslash escapes
+of
+.BR bash (1):
+.PP
+.RS
+.nf
+\fB\e\&u\fP        user name
+\fB\e\&h\fP, \fB\e\&H\fP    host name up to the first \(oq.\(cq, full host name
+\fB\e\&w\fP, \fB\e\&W\fP    working directory (\fB~\fP for $HOME), its last component
+\fB\e\&$\fP        \fB#\fP for root, otherwise \fB$\fP
+\fB\e\&t\fP, \fB\e\&T\fP    time as HH:MM:SS (24 and 12 hours)
+\fB\e\&@\fP, \fB\e\&A\fP    time as HH:MM AM/PM and HH:MM (24 hours)
+\fB\e\&d\fP        date, e.g. Mon Oct 05
+\fB\e\&j\fP        number of jobs
+\fB\e\&l\fP        terminal name
+\fB\e\&s\fP, \fB\e\&v\fP, \fB\e\&V\fP shell name, version (major.minor), full version
+\fB\e\&n\fP, \fB\e\&r\fP    newline, carriage return
+\fB\e\&a\fP, \fB\e\&e\fP    bell, escape
+\fB\e\&\fP\fInnn\fP      character with octal code \fInnn\fP
+\fB\e\e\fP        backslash
+\fB\e[\fP, \fB\e]\fP    ignored (needed by bash around escape sequences)
+.fi
+.RE
+.PP
+Unknown escapes are kept as they are. Then
+.BI $ VAR\fR,\fP
+.BR ${...} ,
+.B $(...)
+and
+.B $((...))
+are expanded;
+.B $?
+is the status of the last command. Control characters in the values of
+escapes and expansions are shown escaped, so colors must be written in
+the prompt itself, e.g.
+.PP
+.RS
+.nf
+PS1=\(aq\e[\ee[1;32m\e]\eu@\eh\e[\ee[0m\e]:\ew\e$ \(aq
+.fi
+.RE
+.PP
+If the prompt cannot be expanded (e.g. because of an unset variable), an
+error is printed and the default prompt is used. Escape sequences other
+than colors (e.g. the window title) are written before the prompt.
 .SH ENVIRONMENT
 .TP
 .B GOSH_HISTFILE
@@ -755,8 +808,18 @@ Path of the history file. If set to an empty value, no history is saved.
 Base directory of the default history file
 .IR $XDG_STATE_HOME/gosh/history .
 .TP
+.B PS1
+Prompt of the interactive shell, see
+.BR "Custom prompts" .
+.TP
+.B PS2
+Prompt for continuation lines (default
+.BR "> " ).
+.TP
 .B GOSH_COLORS
-Prompt colors as colon-separated
+Colors of the default prompt (if
+.B PS1
+is not set) as colon-separated
 .IB key = code
 entries. Keys are
 .B user