feat(builtins): add json command (non-standard, simplified jq)

[chaliy]

Summary

A new, non-standard builtin providing simple JSON operations for the 90% case. jq is powerful but has a learning curve. json would cover common get/set/merge operations with intuitive syntax.

Note: This is a bashkit-specific extension, not a standard command. jq remains available for complex queries.

Proposed Syntax

json <subcommand> [args...] [< input]

Proposed Subcommands

Subcommand	Description
`json get .path`	Extract value at JSON path
`json set .path value`	Set value at JSON path
`json delete .path`	Remove key at JSON path
`json merge file1 file2`	Deep merge two JSON objects
`json keys`	List top-level keys
`json type .path`	Print type (string, number, object, array, bool, null)
`json valid`	Validate JSON (exit 0 if valid, 1 if not)
`json fmt`	Pretty-print JSON
`json compact`	Minify JSON
`json each .array`	Iterate array, output one element per line

Use Cases

# Get a value
echo '{"name":"bashkit","version":"0.1"}' | json get .version
# 0.1

# Set a value
echo '{"name":"bashkit"}' | json set .version "0.2"
# {"name":"bashkit","version":"0.2"}

# Merge configs
json merge defaults.json overrides.json > config.json

# Validate
if echo "$response" | json valid; then
    echo "$response" | json get .data.id
fi

# Iterate array
echo '{"items":[1,2,3]}' | json each .items
# 1
# 2
# 3

# Pipeline with set
cat config.json | json set .database.host "localhost" | json set .database.port 5432 > updated.json

Implementation Notes

• Uses dot notation for paths: .foo.bar[0].baz
• Array access: .items[0], .items[-1] (last), .items[*] (all)
• json get outputs raw value for strings (no quotes), quoted for objects/arrays
• json set with numeric value → JSON number; quoted → JSON string
• Could reuse the existing serde_json dependency
• NOT a replacement for jq — complex filters/transforms should still use jq
• Exit codes: 0 = success, 1 = path not found or invalid JSON

[chaliy]

System Prompt & Progressive Disclosure

Non-standard builtin — LLMs will not know about it. System prompt must teach, but briefly.

llm_hint() — ONE line

fn llm_hint(&self) -> Option<&'static str> {
    Some("json: Simple JSON get/set/merge/each. `echo '{"k":1}' | json get .k` -> 1. Use jq for complex queries.")
}

Progressive disclosure levels

1. System prompt (above hint) — LLM knows subcommands exist
2. json --help — subcommand list with one-liner descriptions
3. json get --help — per-subcommand flags and path notation
4. help json — full man-page with examples

[chaliy]

Documentation Requirements

Since this is a non-standard (invented) builtin, it must be documented in two places:

1. README.md — Add to the "Built-in Commands" table under a new Extensions category row (non-standard bashkit-specific commands, clearly marked as extensions)
2. crates/bashkit/docs/extensions.md — Create or update a dedicated extensions guide covering all invented builtins, with short descriptions, examples, and rationale for why each exists

This ensures users discover these commands and understand they are bashkit-specific, not standard Bash.

[chaliy]

Resolved in PR #585 or #588