codedb_remote: reject empty query on actions that consume it

[justrach]

Problem

`codedb_remote action=search` without a `query` argument silently sends `q=` (empty string) to the remote. The remote returns either an empty result set or a confusing error, and the user can't tell whether their search was genuinely empty or malformed. Same applies to `action=symbol` and `action=outline` on the wiki backend.

Found during triage of open issues while I was looking at #278 and #304 — this is a separate bug the validation I was auditing didn't catch.

Fix

Validate that `query` is non-empty before building URLs, for actions that actually consume it:

• `search` (both backends) → requires query
• `symbol` (wiki backend) → requires query
• `outline` (wiki backend) → requires query (the file path)
• `tree` / `meta` / `policy` → unchanged (these legitimately take no query)

Error messages are specific so the caller knows which field to supply:

```
error: action 'search' requires a non-empty 'query' (the search text)
error: action 'symbol' requires a non-empty 'query' (the identifier name to look up)
error: action 'outline' requires a non-empty 'query' (the file path to outline)
```

Verified via MCP stdio

```
codedb_remote {"repo":"x","action":"search"}
→ error with guidance
codedb_remote {"repo":"x","action":"symbol","backend":"wiki"}
→ error with guidance
codedb_remote {"repo":"sinclairzx81/typebox","action":"tree","backend":"wiki"}
→ succeeds (unchanged, 1.5s)
codedb_remote {"repo":"rust-lang/rust","action":"symbol","query":"HashMap","backend":"wiki"}
→ 25 results (unchanged)
```

Not a regression

The missing-query case was already broken before my wiki backend landed; `codedb_remote action=search` without a query has always silently sent an empty q= to codegraff. This just surfaces the failure with a useful error instead.

🤖 Generated with Claude Code

[github-actions]

Benchmark Regression Report

Threshold: 10.00%

Tool	Base (ns)	Head (ns)	Delta	Status
codedb_bundle	484596	513892	+6.05%	OK
codedb_changes	56459	62166	+10.11%	FAIL
codedb_deps	8717	9449	+8.40%	OK
codedb_edit	4869	5238	+7.58%	OK
codedb_find	65371	72614	+11.08%	FAIL
codedb_hot	98517	100514	+2.03%	OK
codedb_outline	243960	239085	-2.00%	OK
codedb_read	84792	84733	-0.07%	OK
codedb_search	174082	178969	+2.81%	OK
codedb_snapshot	2616112	2602848	-0.51%	OK
codedb_status	220670	211223	-4.28%	OK
codedb_symbol	59472	63317	+6.47%	OK
codedb_tree	82432	81908	-0.64%	OK
codedb_word	69774	67853	-2.75%	OK