docs: add session context for future revisit

SESSION.md

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+# aerc Inline Images — Session Context
+
+## Git State
+- Branch: `feat/inline-images` (12 commits on top of pristine 0.20.0)
+- Remote: `git.sethpc.xyz/Seth/aerc-inline-images`
+- All tests pass, builds clean
+
+## Config State (reverted to stock)
+- `~/.config/aerc/aerc.conf`: `text/html=html-unsafe` (system default)
+- To re-enable images: change to `text/html=html-kitty-unsafe`
+- Custom filter at `~/.local/libexec/aerc/filters/html-kitty` (backup at `.backup/`)
+- Logging disabled (`#log-file=`, `#log-level=info`)
+
+## Key Discovery: Filter Output Interception
+The spec's architecture was wrong. OSC markers are in filter stdout, not pv.source.
+Fix: buffer filter.Stdout in attemptCopy(), run ExtractImages on buffer.
+
+## Key Discovery: Vaxis Sixel Crash
+`vaxis.(*Sixel).Resize()` spawns a goroutine that panics in go-sixel on certain dimensions.
+Cannot be caught with defer/recover (different goroutine). Only affects non-Kitty terminals.
+Kitty protocol path is stable.
+
+## What Needs Work
+1. HTML rendering quality — composite view is bare text, no pager features
+2. Study existing TUI image viewers for better rendering patterns
+3. Token auto-refresh for IMAP OAuth