fix(title): don't let an unrecognised enabled value silently disable titling

[fabiosiqueira]

Follow-up to the auxiliary.title_generation.enabled gate that #66049 landed
(salvaging #37349), for the issue it was fixing: #41744.

The gap

_auto_title_enabled() (agent/title_generator.py:57) reads the knob with
is_truthy_value(title_config.get("enabled"), default=True). That helper answers
"is this truthy?" — the right question for an env var, the wrong one for a config
key whose documented default is on. default=True applies only when the key is
absent (value is None), so every value the truthy set doesn't recognise is
falsy, and the safe direction ends up being a typo in the key, not in the value:

enabled: true    -> True      titling on
enabled: ture    -> 'ture'    titling OFF   <- typo meant to enable, disables instead
enbaled: false   -> None      titling on    <- typo in the key falls through to the default

TRUTHY_STRINGS is {"1", "true", "yes", "on"} (utils.py:19), and YAML parses
an unquoted ture as the string "ture". So a user who mistypes the value while
trying to turn titling on silently loses it, with nothing in the log to say
why — the same failure shape #41744 reported, arriving through the fix for it.
cli-config.yaml.example:634 documents the knob as enabled: true, which is
what makes a value typo plausible in the first place.

The change

Recognise the falsy tokens explicitly and treat anything else as "not a value I
understand": keep the documented default and warn, naming the offending value.
Booleans, the truthy tokens and the falsy tokens all behave exactly as before —
enabled: false, no, off, 0 still disable titling.

Scope

Deliberately at this call site, not in utils.is_truthy_value. The helper is
shared, and env_var_enabled passes default=False, where collapsing
unrecognised into False is correct — the distinction belongs to callers whose
default is True. Seven call sites pass default=True today
(title_generator.py:68, verify_hooks.py:47, cli.py:12762, voice.py:259,
web_routers/tools.py:96, delegate_tool.py:654,
transcription_tools.py:162); this PR fixes the one reported in #41744 rather
than assuming the same answer fits the other six. Happy to follow up on the rest
if you'd like it handled centrally.

Tests

tests/agent/test_title_generator.py — parametrised over recognised truthy
values, recognised falsy values, unrecognised values (asserting titling stays on
and that the warning names the value), and the absent key. The unrecognised
cases fail on current main and pass with the change; the rest pass on both.
32 passed for the file.

— 🤖 Claude Opus 5