singleton: guard the state, not the program's name

The singleton lock protected a filename, not a store. It was keyed on
$EL_SINGLETON_DIR|$TMPDIR|/tmp + /el-singleton-<program>.lock — the
program's NAME and a temp directory — and never consulted the state it
claimed to protect, while its own refusal message read "Refusing to start
a second instance against the same state."

Measured, it failed in both directions. A second engram against a
DIFFERENT data dir was refused, naming the first's pid. And
TMPDIR=/tmp/other let a second engram start against the SAME data dir
with no complaint — the two-writer data-loss condition the guard exists
to prevent, defeated by one environment variable.

Both are one error: the identity of the resource had been replaced by a
label for it.

The lock now lives inside the state it guards —
<state>/.el-singleton-<id>.lock — and the program block says what that
state is. Same directory is the same file is the same inode, so it
contends and there is no TMPDIR left in the key to change. Different
directories are different files, so they don't. Different spellings of
one directory (trailing slash, x/../x, symlink) collapse in the kernel's
own path walk, so they contend without this code comparing strings;
canonicalisation is for the message, never the decision.

`guards:` is an expression so a program can point at the resolver that
already owns its path — guards: engram_resolve_data_dir() — instead of
restating that resolver's default, which is the two-owners defect spec
18.4 exists to prevent. A `singleton:` without `guards:` is now a compile
error; emitting a name-keyed lock instead would be emitting the defect.

Kept: the flock (the kernel drops it on crash and SIGKILL, so there is
still no "delete the lock file to get unstuck" ritual — a stale file
inside a copied data dir is inert), and the holder's pid in the message.
Changed: the message is true. It says "the same state" because the lock
it failed to take is in that state, and it names the state it checked.
An unguardable state (missing, read-only) now refuses rather than
starting unguarded.

Also corrects lang/AGENTS.md's compiler rebuild line, which had gone
stale: linking el_runtime.c alone no longer resolves.

lang/runtime/el_runtime.h

@@ -1091,7 +1091,7 @@ el_val_t  __env_get(el_val_t key);
  * All three are COMPILER-INJECTED at the head of main() — they are not meant to
  * be written by hand, which is the point: the guarantee cannot be forgotten at a
  * call site because there is no call site. */
-el_val_t  el_singleton_acquire(el_val_t id);                    /* §18.1 process identity */
+el_val_t  el_singleton_acquire(el_val_t id, el_val_t state);    /* §18.2 process identity — keyed on the guarded state */
 el_val_t  el_config_declare(el_val_t name, el_val_t type,
                             el_val_t deflt, el_val_t has_default,
                             el_val_t required);                 /* §18.2 config schema    */