PR #10 polish: standardize MCP warning shape + close small coverage gaps

[detour1999]

Follow-up to PR #10. Bundle of small polish items — individually not worth an issue each, together worth one focused sweep.

1. Standardize MCP warning message shape

Warning strings across the MCP surface drift in style:

```
"project lookup failed for project %d: %v" // prose
"activity range unavailable: %v" // terse noun+colon
"tool stats unavailable: %v" // same
"tokens_by_day unavailable: %v" // same
"project %d not found — session references a missing project" // verbose with em-dash
```

Proposal: adopt one shape everywhere — `" unavailable: "`. Concretely:

• `lookupProjectPath`: `"project %d unavailable: not found"` and `"project %d unavailable: %v"`
• Everything else already conforms.

2. Simplify `lookupProjectPath` return type

Currently returns `(string, []string)`. The two error paths each return a slice of length 1, and no caller aggregates. Simpler as `(path string, warning string)` — callers check `if warning != ""`.

3. Fill test gaps

Three PR #10 behaviors either aren't tested or are only tested transitively:

• `list_sessions` and `list_projects` clamp `limit > 100` to 100. `TestListProjects_ReportsHasMoreAndClampsLimit` is named for it but the body only tests `limit=1` and `limit=0`. Add a `limit=200` case.
• `get_stats` emits warnings when its own enrichment queries fail. Currently only exercised transitively via `TestGetAnalytics_PropagatesSummaryWarnings`. Add a direct `TestGetStats_DegradedFieldsWarn` that drops `tool_uses` and asserts `result.warnings` mentions tool stats + `top_tools` is absent from the result.
• `get_stats` omits `first_activity` / `last_activity` / `days_span` when `GetFirstAndLastActivity` fails. No test today.

None of these are broken — they behave as documented in skill docs. Just missing verification.

Not scope

• The bifurcated warnings location and the pagination model split are tracked as separate follow-ups (see other issues filed alongside this one).