feat(cli): add --since, --tail, --json flags to tilt logs

[0xBigBoss]

Summary

Implements #6652 - adds filtering and JSON output capabilities to tilt logs:

• --since: Filter logs by time (e.g., --since 5m, --since 1h)
• --tail: Limit output to last N lines (applies only to initial history when combined with -f)
• --json: Output logs as JSON Lines (JSONL) format
• --json-fields: Configure JSON output fields (minimal, full, or comma-separated)

Usage Examples

# Time-based filtering
tilt logs --since 5m
tilt logs api --since 1h

# Tail limiting
tilt logs --tail 100
tilt logs --tail 50 -f  # Last 50 lines, then stream all new

# JSON output
tilt logs --json
tilt logs --json --json-fields=full
tilt logs --json --json-fields=time,resource,message

Implementation

• Extends LogFilter with Since/Tail fields and time-based filtering
• Adds JSONPrinter for structured JSONL output with configurable fields
• Uses pointer types in JSONLogLine to distinguish "not included" from "empty value"
• LogStreamer tracks isFirstBatch to apply tail only to initial history in follow mode

Follows existing patterns from --level and --source flags (PR #6513).

Test plan

• Unit tests for time-based filtering (TestLogFilterApplyWithSince)
• Unit tests for tail limiting (TestLogFilterApplyWithTail)
• Unit tests for JSON output (TestJSONPrinter*)
• Integration tests for LogStreamer
• go vet passes
• go build succeeds
• Manual testing with tilt logs --help

Closes #6652