fix(profiles): rename silently-ignored fields + wire Phase P in full-opti-v5

[Destynova2]

Summary

The `profiles/full-opti-v5.toml` profile had five field names that serde silently ignored, so the profile shipped with default values for those keys instead of the values written. Discovered while wiring the new Phase P SimHash threshold.

Renames (silently dropped → operational)

Section	Was	Should be
`[router]`	`web_search`	`websearch`
`[cache]`	`ttl_seconds`	`ttl_secs`
`[cache]`	`max_entries`	`max_capacity`
`[budget]`	`warning_threshold_percent`	`warn_at_percent`
`[security]`	`rate_limit_rpm = 120`	`rate_limit_rps = 2` + `rate_limit_burst = 30`

The security split keeps the same sustained rate (2 rps × 60 = 120 rpm) but adds a 30-request burst tolerance — Claude Code routinely fires 5–10 parallel tool calls in a single turn.

Phase P additions

• `cache.simhash_threshold = 4` — slightly more permissive than the default 3 to favor cache hits on paraphrases / whitespace edits without colliding unrelated prompts.
• `cache.max_entry_bytes = 4 MiB` — explicit (was unset, falling back to 2 MiB default).
• `cache.max_capacity` 1000 → 10000 — 10× window, ~20 MiB total memory.
• `cache.ttl_secs` 3600 → 14400 — 4h covers a full Claude Code day.

Validation

```sh
grob --config profiles/full-opti-v5.toml doctor
```
returns `🎉 All checks passed!` and prints `rate_limit=2rps` — proof that the renamed field is now read by the parser (before this PR, the value was silently the default).

Test plan

• `grob doctor` on the patched profile returns success
• Doctor reports `rate_limit=2rps` (confirms the renamed field is parsed)
• CI green (auto-merge enabled below)