feat: split OMIT_IO into OMIT_PROMPTS and OMIT_TOOL_IO

[k-berger]

Summary

• Splits the single OMIT_IO privacy flag into two independent flags: OMIT_PROMPTS (gates user prompts + assistant responses) and OMIT_TOOL_IO (gates tool call inputs + responses). Both default to true, matching the previous OMIT_IO=true default — so out-of-the-box behavior is unchanged.
• Enables the previously-impossible combination: collect tool call arguments and responses while still redacting prompts (OMIT_PROMPTS=true, OMIT_TOOL_IO=false), or the reverse.
• Hard break — OMIT_IO is removed entirely, no alias. Users who explicitly set OMIT_IO=false to enable collection will need to migrate to the new flags; users on defaults are unaffected.

Implementation: contentKeys map in internal/otlp/otlp.go splits into promptContentKeys + toolContentKeys, and the redaction branch in eventAttributes becomes (cfg.OmitPrompts && promptContentKeys[k]) || (cfg.OmitToolIO && toolContentKeys[k]). Wired through Config, cmd/on-event/main.go, the userConfig declarations in plugin.json, the YAML frontmatter parser in scripts/on-event.sh, and DASH0_* env var fallbacks. README updated across the privacy defaults table, configuration tables, env var table, config file examples, and attribute footnotes.

Test plan

• go build ./... clean
• go test ./... — all packages pass (unit + integration)
• Tests updated: split TestSendLogOmitIO into three sub-tests (both flags, prompts-only, tool-IO-only); split tool/LLM span tests in trace_test.go to verify each flag is wired to the right span type; renamed *SurvivesOmitIO extraction tests to *SurvivesOmitToolIO and switched their env var
• e2e test env updated to set both CLAUDE_PLUGIN_OPTION_OMIT_PROMPTS=false and CLAUDE_PLUGIN_OPTION_OMIT_TOOL_IO=false
• Manual: install plugin, set OMIT_PROMPTS=true + OMIT_TOOL_IO=false, confirm only tool I/O reaches Dash0
• Release notes flag the breaking rename for anyone who was using OMIT_IO=false