docs: fix block end delimiter, slugify signature, and stale repo refs

[httpdss]

Three documented behaviours did not match the implementation.

1. Block end delimiter was written as %@} in two places.
   template_renderer.py sets block_end_string='@%}'. Using %@} raises
   TemplateSyntaxError. template-variables.md contradicted itself: correct
   in the "Custom Delimiters" summary, wrong in "Block Syntax" below it.

2. slugify was documented as taking an optional separator argument.
   filters.py defines slugify(value) with no second parameter, so
   slugify(separator="_") raises TypeError. Replaced with the actual
   behaviour and the | slugify | replace("-", "_") idiom. Also noted that
   underscores are stripped rather than converted (My_Project -> myproject).

3. Two default_branch examples referenced httpdss/struct, the old
   repository name.

Also adds the one global missing from the reference (current_repo()),
documents to_json's indent argument, and warns that uuid() and now()
are non-deterministic — a file using either always appears in
generate --dry-run --diff, which silently ruins that diff as a drift check.

How this was verified

import re
from jinja2 import Environment

def slugify(value):                      # copy of structkit/filters.py
    value = value.lower()
    value = re.sub(r'\s+', '-', value)
    value = re.sub(r'[^a-z0-9-]', '', value)
    return value

env = Environment(
    trim_blocks=True,
    block_start_string='{%@', block_end_string='@%}',
    variable_start_string='{{@', variable_end_string='@}}',
    comment_start_string='{#@', comment_end_string='@#}',
)
env.filters['slugify'] = slugify

print(env.from_string('{%@ if x @%}YES{%@ endif @%}').render(x=True))   # YES
try:
    env.from_string('{%@ if x %@}YES{%@ endif %@}').render(x=True)
    print('BUG: %@} parsed')
except Exception as e:
    print('%@} ->', type(e).__name__)                                   # TemplateSyntaxError
print(env.from_string('{{@ n | slugify @}}').render(n="My_Project"))    # myproject
try:
    env.from_string('{{@ n | slugify(separator="_") @}}').render(n="a b")
    print('BUG: separator accepted')
except Exception as e:
    print('separator ->', type(e).__name__)                             # TypeError

Output:

YES
%@} -> TemplateSyntaxError
myproject
separator -> TypeError